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 <div class="section" id="expressions">
<h1>6. Expressions<a class="headerlink" href="#expressions" title="Permalink to this headline">¶</a></h1>
This chapter explains the meaning of the elements of expressions in Python.
Syntax Notes: In this and the following chapters, extended BNF notation will
be used to describe syntax, not lexical analysis. When (one alternative of) a
syntax rule has the form
<pre>
name ::= <code class="xref docutils literal notranslate">othername</code>
</pre>
and no semantics are given, the semantics of this form of <code class="docutils literal notranslate">name</code> are the same
as for <code class="docutils literal notranslate">othername</code>.
<div class="section" id="arithmetic-conversions">
<h2>6.1. Arithmetic conversions<a class="headerlink" href="#arithmetic-conversions" title="Permalink to this headline">¶</a></h2>
When a description of an arithmetic operator below uses the phrase “the numeric
arguments are converted to a common type,” this means that the operator
implementation for built-in types works as follows:
<ul class="simple">
<li>If either argument is a complex number, the other is converted to complex;</li>
<li>otherwise, if either argument is a floating point number, the other is
converted to floating point;</li>
<li>otherwise, both must be integers and no conversion is necessary.</li>
</ul>
Some additional rules apply for certain operators (e.g., a string as a left
argument to the ‘%’ operator). Extensions must define their own conversion
behavior.
</div>
<div class="section" id="atoms">
<h2>6.2. Atoms<a class="headerlink" href="#atoms" title="Permalink to this headline">¶</a></h2>
Atoms are the most basic elements of expressions. The simplest atoms are
identifiers or literals. Forms enclosed in parentheses, brackets or braces are
also categorized syntactically as atoms. The syntax for atoms is:
<pre>
atom ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a> | <a class="reference internal" href="#grammar-token-literal"><code class="xref docutils literal notranslate">literal</code></a> | <a class="reference internal" href="#grammar-token-enclosure"><code class="xref docutils literal notranslate">enclosure</code></a>
enclosure ::= <a class="reference internal" href="#grammar-token-parenth_form"><code class="xref docutils literal notranslate">parenth_form</code></a> | <a class="reference internal" href="#grammar-token-list_display"><code class="xref docutils literal notranslate">list_display</code></a> | <a class="reference internal" href="#grammar-token-dict_display"><code class="xref docutils literal notranslate">dict_display</code></a> | <a class="reference internal" href="#grammar-token-set_display"><code class="xref docutils literal notranslate">set_display</code></a>
 | <a class="reference internal" href="#grammar-token-generator_expression"><code class="xref docutils literal notranslate">generator_expression</code></a> | <a class="reference internal" href="#grammar-token-yield_atom"><code class="xref docutils literal notranslate">yield_atom</code></a>
</pre>
<div class="section" id="atom-identifiers">
<h3>6.2.1. Identifiers (Names)<a class="headerlink" href="#atom-identifiers" title="Permalink to this headline">¶</a></h3>
An identifier occurring as an atom is a name. See section <a class="reference internal" href="lexical_analysis.html#identifiers">Identifiers and keywords</a>
for lexical definition and section <a class="reference internal" href="executionmodel.html#naming">Naming and binding</a> for documentation of naming and
binding.
When the name is bound to an object, evaluation of the atom yields that object.
When a name is not bound, an attempt to evaluate it raises a <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate">NameError</code></a>
exception.
Private name mangling: When an identifier that textually occurs in a class
definition begins with two or more underscore characters and does not end in two
or more underscores, it is considered a private name of that class.
Private names are transformed to a longer form before code is generated for
them. The transformation inserts the class name, with leading underscores
removed and a single underscore inserted, in front of the name. For example,
the identifier <code class="docutils literal notranslate">__spam</code> occurring in a class named <code class="docutils literal notranslate">Ham</code> will be transformed
to <code class="docutils literal notranslate">_Ham__spam</code>. This transformation is independent of the syntactical
context in which the identifier is used. If the transformed name is extremely
long (longer than 255 characters), implementation defined truncation may happen.
If the class name consists only of underscores, no transformation is done.
</div>
<div class="section" id="literals">
<h3>6.2.2. Literals<a class="headerlink" href="#literals" title="Permalink to this headline">¶</a></h3>
Python supports string and bytes literals and various numeric literals:
<pre>
literal ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-stringliteral"><code class="xref docutils literal notranslate">stringliteral</code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-bytesliteral"><code class="xref docutils literal notranslate">bytesliteral</code></a>
 | <a class="reference internal" href="lexical_analysis.html#grammar-token-integer"><code class="xref docutils literal notranslate">integer</code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-floatnumber"><code class="xref docutils literal notranslate">floatnumber</code></a> | <a class="reference internal" href="lexical_analysis.html#grammar-token-imagnumber"><code class="xref docutils literal notranslate">imagnumber</code></a>
</pre>
Evaluation of a literal yields an object of the given type (string, bytes,
integer, floating point number, complex number) with the given value. The value
may be approximated in the case of floating point and imaginary (complex)
literals. See section <a class="reference internal" href="lexical_analysis.html#literals">Literals</a> for details.
All literals correspond to immutable data types, and hence the object’s identity
is less important than its value. Multiple evaluations of literals with the
same value (either the same occurrence in the program text or a different
occurrence) may obtain the same object or a different object with the same
value.
</div>
<div class="section" id="parenthesized-forms">
<h3>6.2.3. Parenthesized forms<a class="headerlink" href="#parenthesized-forms" title="Permalink to this headline">¶</a></h3>
A parenthesized form is an optional expression list enclosed in parentheses:
<pre>
parenth_form ::= &quot;(&quot; [<a class="reference internal" href="#grammar-token-starred_expression"><code class="xref docutils literal notranslate">starred_expression</code></a>] &quot;)&quot;
</pre>
A parenthesized expression list yields whatever that expression list yields: if
the list contains at least one comma, it yields a tuple; otherwise, it yields
the single expression that makes up the expression list.
An empty pair of parentheses yields an empty tuple object. Since tuples are
immutable, the rules for literals apply (i.e., two occurrences of the empty
tuple may or may not yield the same object).
Note that tuples are not formed by the parentheses, but rather by use of the
comma operator. The exception is the empty tuple, for which parentheses are
required — allowing unparenthesized “nothing” in expressions would cause
ambiguities and allow common typos to pass uncaught.
</div>
<div class="section" id="displays-for-lists-sets-and-dictionaries">
<h3>6.2.4. Displays for lists, sets and dictionaries<a class="headerlink" href="#displays-for-lists-sets-and-dictionaries" title="Permalink to this headline">¶</a></h3>
For constructing a list, a set or a dictionary Python provides special syntax
called “displays”, each of them in two flavors:
<ul class="simple">
<li>either the container contents are listed explicitly, or</li>
<li>they are computed via a set of looping and filtering instructions, called a
comprehension.</li>
</ul>
Common syntax elements for comprehensions are:
<pre>
comprehension ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate">comp_for</code></a>
comp_for ::= [ASYNC] &quot;for&quot; <a class="reference internal" href="simple_stmts.html#grammar-token-target_list"><code class="xref docutils literal notranslate">target_list</code></a> &quot;in&quot; <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate">or_test</code></a> [<a class="reference internal" href="#grammar-token-comp_iter"><code class="xref docutils literal notranslate">comp_iter</code></a>]
comp_iter ::= <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate">comp_for</code></a> | <a class="reference internal" href="#grammar-token-comp_if"><code class="xref docutils literal notranslate">comp_if</code></a>
comp_if ::= &quot;if&quot; <a class="reference internal" href="#grammar-token-expression_nocond"><code class="xref docutils literal notranslate">expression_nocond</code></a> [<a class="reference internal" href="#grammar-token-comp_iter"><code class="xref docutils literal notranslate">comp_iter</code></a>]
</pre>
The comprehension consists of a single expression followed by at least one
<a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> clause and zero or more <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or <a class="reference internal" href="compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> clauses.
In this case, the elements of the new container are those that would be produced
by considering each of the <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or <a class="reference internal" href="compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> clauses a block,
nesting from left to right, and evaluating the expression to produce an element
each time the innermost block is reached.
Note that the comprehension is executed in a separate scope, so names assigned
to in the target list don’t “leak” into the enclosing scope.
Since Python 3.6, in an <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate">async def</code></a> function, an <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a>
clause may be used to iterate over a <a class="reference internal" href="../glossary.html#term-asynchronous-iterator">asynchronous iterator</a>.
A comprehension in an <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate">async def</code></a> function may consist of either a
<a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> clause following the leading
expression, may contain additional <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a>
clauses, and may also use <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a> expressions.
If a comprehension contains either <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> clauses
or <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a> expressions it is called an
asynchronous comprehension. An asynchronous comprehension may
suspend the execution of the coroutine function in which it appears.
See also <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0530">PEP 530</a>.
</div>
<div class="section" id="list-displays">
<h3>6.2.5. List displays<a class="headerlink" href="#list-displays" title="Permalink to this headline">¶</a></h3>
A list display is a possibly empty series of expressions enclosed in square
brackets:
<pre>
list_display ::= &quot;[&quot; [<a class="reference internal" href="#grammar-token-starred_list"><code class="xref docutils literal notranslate">starred_list</code></a> | <a class="reference internal" href="#grammar-token-comprehension"><code class="xref docutils literal notranslate">comprehension</code></a>] &quot;]&quot;
</pre>
A list display yields a new list object, the contents being specified by either
a list of expressions or a comprehension. When a comma-separated list of
expressions is supplied, its elements are evaluated from left to right and
placed into the list object in that order. When a comprehension is supplied,
the list is constructed from the elements resulting from the comprehension.
</div>
<div class="section" id="set-displays">
<h3>6.2.6. Set displays<a class="headerlink" href="#set-displays" title="Permalink to this headline">¶</a></h3>
A set display is denoted by curly braces and distinguishable from dictionary
displays by the lack of colons separating keys and values:
<pre>
set_display ::= &quot;{&quot; (<a class="reference internal" href="#grammar-token-starred_list"><code class="xref docutils literal notranslate">starred_list</code></a> | <a class="reference internal" href="#grammar-token-comprehension"><code class="xref docutils literal notranslate">comprehension</code></a>) &quot;}&quot;
</pre>
A set display yields a new mutable set object, the contents being specified by
either a sequence of expressions or a comprehension. When a comma-separated
list of expressions is supplied, its elements are evaluated from left to right
and added to the set object. When a comprehension is supplied, the set is
constructed from the elements resulting from the comprehension.
An empty set cannot be constructed with <code class="docutils literal notranslate">{}</code>; this literal constructs an empty
dictionary.
</div>
<div class="section" id="dictionary-displays">
<h3>6.2.7. Dictionary displays<a class="headerlink" href="#dictionary-displays" title="Permalink to this headline">¶</a></h3>
A dictionary display is a possibly empty series of key/datum pairs enclosed in
curly braces:
<pre>
dict_display ::= &quot;{&quot; [<a class="reference internal" href="#grammar-token-key_datum_list"><code class="xref docutils literal notranslate">key_datum_list</code></a> | <a class="reference internal" href="#grammar-token-dict_comprehension"><code class="xref docutils literal notranslate">dict_comprehension</code></a>] &quot;}&quot;
key_datum_list ::= <a class="reference internal" href="#grammar-token-key_datum"><code class="xref docutils literal notranslate">key_datum</code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-key_datum"><code class="xref docutils literal notranslate">key_datum</code></a>)* [&quot;,&quot;]
key_datum ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> | &quot;**&quot; <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate">or_expr</code></a>
dict_comprehension ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> &quot;:&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate">comp_for</code></a>
</pre>
A dictionary display yields a new dictionary object.
If a comma-separated sequence of key/datum pairs is given, they are evaluated
from left to right to define the entries of the dictionary: each key object is
used as a key into the dictionary to store the corresponding datum. This means
that you can specify the same key multiple times in the key/datum list, and the
final dictionary’s value for that key will be the last one given.
A double asterisk <code class="docutils literal notranslate">**</code> denotes dictionary unpacking.
Its operand must be a <a class="reference internal" href="../glossary.html#term-mapping">mapping</a>. Each mapping item is added
to the new dictionary. Later values replace values already set by
earlier key/datum pairs and earlier dictionary unpackings.
<div class="versionadded">
New in version 3.5: Unpacking into dictionary displays, originally proposed by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0448">PEP 448</a>.
</div>
A dict comprehension, in contrast to list and set comprehensions, needs two
expressions separated with a colon followed by the usual “for” and “if” clauses.
When the comprehension is run, the resulting key and value elements are inserted
in the new dictionary in the order they are produced.
Restrictions on the types of the key values are listed earlier in section
<a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>. (To summarize, the key type should be <a class="reference internal" href="../glossary.html#term-hashable">hashable</a>, which excludes
all mutable objects.) Clashes between duplicate keys are not detected; the last
datum (textually rightmost in the display) stored for a given key value
prevails.
</div>
<div class="section" id="generator-expressions">
<h3>6.2.8. Generator expressions<a class="headerlink" href="#generator-expressions" title="Permalink to this headline">¶</a></h3>
A generator expression is a compact generator notation in parentheses:
<pre>
generator_expression ::= &quot;(&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> <a class="reference internal" href="#grammar-token-comp_for"><code class="xref docutils literal notranslate">comp_for</code></a> &quot;)&quot;
</pre>
A generator expression yields a new generator object. Its syntax is the same as
for comprehensions, except that it is enclosed in parentheses instead of
brackets or curly braces.
Variables used in the generator expression are evaluated lazily when the
<a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a> method is called for the generator object (in the same
fashion as normal generators). However, the leftmost <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> clause is
immediately evaluated, so that an error produced by it can be seen before any
other possible error in the code that handles the generator expression.
Subsequent <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> clauses cannot be evaluated immediately since they
may depend on the previous <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> loop. For example: <code class="docutils literal notranslate">(x*y for x in
range(10) for y in bar(x))</code>.
The parentheses can be omitted on calls with only one argument. See section
<a class="reference internal" href="#calls">Calls</a> for details.
Since Python 3.6, if the generator appears in an <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate">async def</code></a> function,
then <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> clauses and <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a> expressions are permitted
as with an asynchronous comprehension. If a generator expression
contains either <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> clauses or <a class="reference internal" href="#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a> expressions
it is called an asynchronous generator expression.
An asynchronous generator expression yields a new asynchronous
generator object, which is an asynchronous iterator
(see <a class="reference internal" href="datamodel.html#async-iterators">Asynchronous Iterators</a>).
</div>
<div class="section" id="yield-expressions">
<h3>6.2.9. Yield expressions<a class="headerlink" href="#yield-expressions" title="Permalink to this headline">¶</a></h3>
<pre id="index-19">
yield_atom ::= &quot;(&quot; <a class="reference internal" href="#grammar-token-yield_expression"><code class="xref docutils literal notranslate">yield_expression</code></a> &quot;)&quot;
yield_expression ::= &quot;yield&quot; [<a class="reference internal" href="#grammar-token-expression_list"><code class="xref docutils literal notranslate">expression_list</code></a> | &quot;from&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>]
</pre>
The yield expression is used when defining a <a class="reference internal" href="../glossary.html#term-generator">generator</a> function
or an <a class="reference internal" href="../glossary.html#term-asynchronous-generator">asynchronous generator</a> function and
thus can only be used in the body of a function definition. Using a yield
expression in a function’s body causes that function to be a generator,
and using it in an <a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate">async def</code></a> function’s body causes that
coroutine function to be an asynchronous generator. For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def gen(): # defines a generator function
 yield 123

async def agen(): # defines an asynchronous generator function (PEP 525)
 yield 123
</pre></div>
</div>
Generator functions are described below, while asynchronous generator
functions are described separately in section
<a class="reference internal" href="#asynchronous-generator-functions">Asynchronous generator functions</a>.
When a generator function is called, it returns an iterator known as a
generator. That generator then controls the execution of the generator function.
The execution starts when one of the generator’s methods is called. At that
time, the execution proceeds to the first yield expression, where it is
suspended again, returning the value of <a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate">expression_list</code></a> to the generator’s
caller. By suspended, we mean that all local state is retained, including the
current bindings of local variables, the instruction pointer, the internal
evaluation stack, and the state of any exception handling. When the execution
is resumed by calling one of the
generator’s methods, the function can proceed exactly as if the yield expression
were just another external call. The value of the yield expression after
resuming depends on the method which resumed the execution. If
<a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a> is used (typically via either a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> or
the <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate">next()</code></a> builtin) then the result is <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>. Otherwise, if
<a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate">send()</code></a> is used, then the result will be the value passed in to
that method.
All of this makes generator functions quite similar to coroutines; they yield
multiple times, they have more than one entry point and their execution can be
suspended. The only difference is that a generator function cannot control
where the execution should continue after it yields; the control is always
transferred to the generator’s caller.
Yield expressions are allowed anywhere in a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> construct. If the
generator is not resumed before it is
finalized (by reaching a zero reference count or by being garbage collected),
the generator-iterator’s <a class="reference internal" href="#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate">close()</code></a> method will be called,
allowing any pending <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clauses to execute.
When <code class="docutils literal notranslate">yield from &lt;expr&gt;</code> is used, it treats the supplied expression as
a subiterator. All values produced by that subiterator are passed directly
to the caller of the current generator’s methods. Any values passed in with
<a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate">send()</code></a> and any exceptions passed in with
<a class="reference internal" href="#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate">throw()</code></a> are passed to the underlying iterator if it has the
appropriate methods. If this is not the case, then <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate">send()</code></a>
will raise <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a> or <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>, while
<a class="reference internal" href="#generator.throw" title="generator.throw"><code class="xref py py-meth docutils literal notranslate">throw()</code></a> will just raise the passed in exception immediately.
When the underlying iterator is complete, the <code class="xref py py-attr docutils literal notranslate">value</code>
attribute of the raised <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> instance becomes the value of
the yield expression. It can be either set explicitly when raising
<a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a>, or automatically when the sub-iterator is a generator
(by returning a value from the sub-generator).
<blockquote>
<div><div class="versionchanged">
Changed in version 3.3: Added <code class="docutils literal notranslate">yield from &lt;expr&gt;</code> to delegate control flow to a subiterator.
</div>
</div></blockquote>
The parentheses may be omitted when the yield expression is the sole expression
on the right hand side of an assignment statement.
<div class="admonition seealso">
See also
<dl class="last docutils">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0255">PEP 255</a> - Simple Generators</dt>
<dd>The proposal for adding generators and the <a class="reference internal" href="simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate">yield</code></a> statement to Python.</dd>
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0342">PEP 342</a> - Coroutines via Enhanced Generators</dt>
<dd>The proposal to enhance the API and syntax of generators, making them
usable as simple coroutines.</dd>
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0380">PEP 380</a> - Syntax for Delegating to a Subgenerator</dt>
<dd>The proposal to introduce the <code class="xref std std-token docutils literal notranslate">yield_from</code> syntax, making delegation
to sub-generators easy.</dd>
</dl>
</div>
<div class="section" id="generator-iterator-methods">
<h4>6.2.9.1. Generator-iterator methods<a class="headerlink" href="#generator-iterator-methods" title="Permalink to this headline">¶</a></h4>
This subsection describes the methods of a generator iterator. They can
be used to control the execution of a generator function.
Note that calling any of the generator methods below when the generator
is already executing raises a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> exception.
<dl class="method">
<dt id="generator.__next__">
<code class="descclassname">generator.</code><code class="descname">__next__</code>()<a class="headerlink" href="#generator.__next__" title="Permalink to this definition">¶</a></dt>
<dd>Starts the execution of a generator function or resumes it at the last
executed yield expression. When a generator function is resumed with a
<a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a> method, the current yield expression always
evaluates to <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>. The execution then continues to the next yield
expression, where the generator is suspended again, and the value of the
<a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate">expression_list</code></a> is returned to <a class="reference internal" href="#generator.__next__" title="generator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a>’s caller. If the
generator exits without yielding another value, a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a>
exception is raised.
This method is normally called implicitly, e.g. by a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> loop, or
by the built-in <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate">next()</code></a> function.
</dd></dl>

<dl class="method">
<dt id="generator.send">
<code class="descclassname">generator.</code><code class="descname">send</code>(value)<a class="headerlink" href="#generator.send" title="Permalink to this definition">¶</a></dt>
<dd>Resumes the execution and “sends” a value into the generator function. The
value argument becomes the result of the current yield expression. The
<a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate">send()</code></a> method returns the next value yielded by the generator, or
raises <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> if the generator exits without yielding another
value. When <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate">send()</code></a> is called to start the generator, it must be called
with <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> as the argument, because there is no yield expression that
could receive the value.
</dd></dl>

<dl class="method">
<dt id="generator.throw">
<code class="descclassname">generator.</code><code class="descname">throw</code>(type[, value[, traceback]])<a class="headerlink" href="#generator.throw" title="Permalink to this definition">¶</a></dt>
<dd>Raises an exception of type <code class="docutils literal notranslate">type</code> at the point where the generator was paused,
and returns the next value yielded by the generator function. If the generator
exits without yielding another value, a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> exception is
raised. If the generator function does not catch the passed-in exception, or
raises a different exception, then that exception propagates to the caller.
</dd></dl>

<dl class="method">
<dt id="generator.close">
<code class="descclassname">generator.</code><code class="descname">close</code>()<a class="headerlink" href="#generator.close" title="Permalink to this definition">¶</a></dt>
<dd>Raises a <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate">GeneratorExit</code></a> at the point where the generator function was
paused. If the generator function then exits gracefully, is already closed,
or raises <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate">GeneratorExit</code></a> (by not catching the exception), close
returns to its caller. If the generator yields a value, a
<a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</code></a> is raised. If the generator raises any other exception,
it is propagated to the caller. <a class="reference internal" href="#generator.close" title="generator.close"><code class="xref py py-meth docutils literal notranslate">close()</code></a> does nothing if the generator
has already exited due to an exception or normal exit.
</dd></dl>

</div>
<div class="section" id="examples">
<h4>6.2.9.2. Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h4>
Here is a simple example that demonstrates the behavior of generators and
generator functions:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def echo(value=None):
... print(&quot;Execution starts when &#39;next()&#39; is called for the first time.&quot;)
... try:
... while True:
... try:
... value = (yield value)
... except Exception as e:
... value = e
... finally:
... print(&quot;Don&#39;t forget to clean up when &#39;close()&#39; is called.&quot;)
...
&gt;&gt;&gt; generator = echo(1)
&gt;&gt;&gt; print(next(generator))
Execution starts when &#39;next()&#39; is called for the first time.
1
&gt;&gt;&gt; print(next(generator))
None
&gt;&gt;&gt; print(generator.send(2))
2
&gt;&gt;&gt; generator.throw(TypeError, &quot;spam&quot;)
TypeError(&#39;spam&#39;,)
&gt;&gt;&gt; generator.close()
Don&#39;t forget to clean up when &#39;close()&#39; is called.
</pre></div>
</div>
For examples using <code class="docutils literal notranslate">yield from</code>, see <a class="reference internal" href="../whatsnew/3.3.html#pep-380">PEP 380: Syntax for Delegating to a Subgenerator</a> in “What’s New in
Python.”
</div>
<div class="section" id="asynchronous-generator-functions">
<h4>6.2.9.3. Asynchronous generator functions<a class="headerlink" href="#asynchronous-generator-functions" title="Permalink to this headline">¶</a></h4>
The presence of a yield expression in a function or method defined using
<a class="reference internal" href="compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate">async def</code></a> further defines the function as a
<a class="reference internal" href="../glossary.html#term-asynchronous-generator">asynchronous generator</a> function.
When an asynchronous generator function is called, it returns an
asynchronous iterator known as an asynchronous generator object.
That object then controls the execution of the generator function.
An asynchronous generator object is typically used in an
<a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> statement in a coroutine function analogously to
how a generator object would be used in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> statement.
Calling one of the asynchronous generator’s methods returns an
<a class="reference internal" href="../glossary.html#term-awaitable">awaitable</a> object, and the execution starts when this object
is awaited on. At that time, the execution proceeds to the first yield
expression, where it is suspended again, returning the value of
<a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate">expression_list</code></a> to the awaiting coroutine. As with a generator,
suspension means that all local state is retained, including the
current bindings of local variables, the instruction pointer, the internal
evaluation stack, and the state of any exception handling. When the execution
is resumed by awaiting on the next object returned by the asynchronous
generator’s methods, the function can proceed exactly as if the yield
expression were just another external call. The value of the yield expression
after resuming depends on the method which resumed the execution. If
<a class="reference internal" href="#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate">__anext__()</code></a> is used then the result is <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>. Otherwise, if
<a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate">asend()</code></a> is used, then the result will be the value passed in to
that method.
In an asynchronous generator function, yield expressions are allowed anywhere
in a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> construct. However, if an asynchronous generator is not
resumed before it is finalized (by reaching a zero reference count or by
being garbage collected), then a yield expression within a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>
construct could result in a failure to execute pending <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a>
clauses. In this case, it is the responsibility of the event loop or
scheduler running the asynchronous generator to call the asynchronous
generator-iterator’s <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate">aclose()</code></a> method and run the resulting
coroutine object, thus allowing any pending <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clauses
to execute.
To take care of finalization, an event loop should define
a finalizer function which takes an asynchronous generator-iterator
and presumably calls <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate">aclose()</code></a> and executes the coroutine.
This finalizer may be registered by calling <a class="reference internal" href="../library/sys.html#sys.set_asyncgen_hooks" title="sys.set_asyncgen_hooks"><code class="xref py py-func docutils literal notranslate">sys.set_asyncgen_hooks()</code></a>.
When first iterated over, an asynchronous generator-iterator will store the
registered finalizer to be called upon finalization. For a reference example
of a finalizer method see the implementation of
<code class="docutils literal notranslate">asyncio.Loop.shutdown_asyncgens</code> in <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/asyncio/base_events.py">Lib/asyncio/base_events.py</a>.
The expression <code class="docutils literal notranslate">yield from &lt;expr&gt;</code> is a syntax error when used in an
asynchronous generator function.
</div>
<div class="section" id="asynchronous-generator-iterator-methods">
<h4>6.2.9.4. Asynchronous generator-iterator methods<a class="headerlink" href="#asynchronous-generator-iterator-methods" title="Permalink to this headline">¶</a></h4>
This subsection describes the methods of an asynchronous generator iterator,
which are used to control the execution of a generator function.
<dl class="method">
<dt id="agen.__anext__">
coroutine <code class="descclassname">agen.</code><code class="descname">__anext__</code>()<a class="headerlink" href="#agen.__anext__" title="Permalink to this definition">¶</a></dt>
<dd>Returns an awaitable which when run starts to execute the asynchronous
generator or resumes it at the last executed yield expression. When an
asynchronous generator function is resumed with a <a class="reference internal" href="#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate">__anext__()</code></a>
method, the current yield expression always evaluates to <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> in
the returned awaitable, which when run will continue to the next yield
expression. The value of the <a class="reference internal" href="#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate">expression_list</code></a> of the yield
expression is the value of the <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> exception raised by
the completing coroutine. If the asynchronous generator exits without
yielding another value, the awaitable instead raises an
<a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate">StopAsyncIteration</code></a> exception, signalling that the asynchronous
iteration has completed.
This method is normally called implicitly by a <a class="reference internal" href="compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> loop.
</dd></dl>

<dl class="method">
<dt id="agen.asend">
coroutine <code class="descclassname">agen.</code><code class="descname">asend</code>(value)<a class="headerlink" href="#agen.asend" title="Permalink to this definition">¶</a></dt>
<dd>Returns an awaitable which when run resumes the execution of the
asynchronous generator. As with the <a class="reference internal" href="#generator.send" title="generator.send"><code class="xref py py-meth docutils literal notranslate">send()</code></a> method for a
generator, this “sends” a value into the asynchronous generator function,
and the value argument becomes the result of the current yield expression.
The awaitable returned by the <a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate">asend()</code></a> method will return the next
value yielded by the generator as the value of the raised
<a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a>, or raises <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate">StopAsyncIteration</code></a> if the
asynchronous generator exits without yielding another value. When
<a class="reference internal" href="#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate">asend()</code></a> is called to start the asynchronous
generator, it must be called with <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> as the argument,
because there is no yield expression that could receive the value.
</dd></dl>

<dl class="method">
<dt id="agen.athrow">
coroutine <code class="descclassname">agen.</code><code class="descname">athrow</code>(type[, value[, traceback]])<a class="headerlink" href="#agen.athrow" title="Permalink to this definition">¶</a></dt>
<dd>Returns an awaitable that raises an exception of type <code class="docutils literal notranslate">type</code> at the point
where the asynchronous generator was paused, and returns the next value
yielded by the generator function as the value of the raised
<a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> exception. If the asynchronous generator exits
without yielding another value, an <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate">StopAsyncIteration</code></a> exception is
raised by the awaitable.
If the generator function does not catch the passed-in exception, or
raises a different exception, then when the awaitable is run that exception
propagates to the caller of the awaitable.
</dd></dl>

<dl class="method">
<dt id="agen.aclose">
coroutine <code class="descclassname">agen.</code><code class="descname">aclose</code>()<a class="headerlink" href="#agen.aclose" title="Permalink to this definition">¶</a></dt>
<dd>Returns an awaitable that when run will throw a <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate">GeneratorExit</code></a> into
the asynchronous generator function at the point where it was paused.
If the asynchronous generator function then exits gracefully, is already
closed, or raises <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate">GeneratorExit</code></a> (by not catching the exception),
then the returned awaitable will raise a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate">StopIteration</code></a> exception.
Any further awaitables returned by subsequent calls to the asynchronous
generator will raise a <a class="reference internal" href="../library/exceptions.html#StopAsyncIteration" title="StopAsyncIteration"><code class="xref py py-exc docutils literal notranslate">StopAsyncIteration</code></a> exception. If the
asynchronous generator yields a value, a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</code></a> is raised
by the awaitable. If the asynchronous generator raises any other exception,
it is propagated to the caller of the awaitable. If the asynchronous
generator has already exited due to an exception or normal exit, then
further calls to <a class="reference internal" href="#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate">aclose()</code></a> will return an awaitable that does nothing.
</dd></dl>

</div>
</div>
</div>
<div class="section" id="primaries">
<h2>6.3. Primaries<a class="headerlink" href="#primaries" title="Permalink to this headline">¶</a></h2>
Primaries represent the most tightly bound operations of the language. Their
syntax is:
<pre>
primary ::= <a class="reference internal" href="#grammar-token-atom"><code class="xref docutils literal notranslate">atom</code></a> | <a class="reference internal" href="#grammar-token-attributeref"><code class="xref docutils literal notranslate">attributeref</code></a> | <a class="reference internal" href="#grammar-token-subscription"><code class="xref docutils literal notranslate">subscription</code></a> | <a class="reference internal" href="#grammar-token-slicing"><code class="xref docutils literal notranslate">slicing</code></a> | <a class="reference internal" href="#grammar-token-call"><code class="xref docutils literal notranslate">call</code></a>
</pre>
<div class="section" id="attribute-references">
<h3>6.3.1. Attribute references<a class="headerlink" href="#attribute-references" title="Permalink to this headline">¶</a></h3>
An attribute reference is a primary followed by a period and a name:
<pre>
attributeref ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate">primary</code></a> &quot;.&quot; <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a>
</pre>
The primary must evaluate to an object of a type that supports attribute
references, which most objects do. This object is then asked to produce the
attribute whose name is the identifier. This production can be customized by
overriding the <a class="reference internal" href="datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate">__getattr__()</code></a> method. If this attribute is not available,
the exception <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a> is raised. Otherwise, the type and value of
the object produced is determined by the object. Multiple evaluations of the
same attribute reference may yield different objects.
</div>
<div class="section" id="subscriptions">
<h3>6.3.2. Subscriptions<a class="headerlink" href="#subscriptions" title="Permalink to this headline">¶</a></h3>
A subscription selects an item of a sequence (string, tuple or list) or mapping
(dictionary) object:
<pre>
subscription ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate">primary</code></a> &quot;[&quot; <a class="reference internal" href="#grammar-token-expression_list"><code class="xref docutils literal notranslate">expression_list</code></a> &quot;]&quot;
</pre>
The primary must evaluate to an object that supports subscription (lists or
dictionaries for example). User-defined objects can support subscription by
defining a <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a> method.
For built-in objects, there are two types of objects that support subscription:
If the primary is a mapping, the expression list must evaluate to an object
whose value is one of the keys of the mapping, and the subscription selects the
value in the mapping that corresponds to that key. (The expression list is a
tuple except if it has exactly one item.)
If the primary is a sequence, the expression list must evaluate to an integer
or a slice (as discussed in the following section).
The formal syntax makes no special provision for negative indices in
sequences; however, built-in sequences all provide a <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a>
method that interprets negative indices by adding the length of the sequence
to the index (so that <code class="docutils literal notranslate">x[-1]</code> selects the last item of <code class="docutils literal notranslate">x</code>). The
resulting value must be a nonnegative integer less than the number of items in
the sequence, and the subscription selects the item whose index is that value
(counting from zero). Since the support for negative indices and slicing
occurs in the object’s <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a> method, subclasses overriding
this method will need to explicitly add that support.
A string’s items are characters. A character is not a separate data type but a
string of exactly one character.
</div>
<div class="section" id="slicings">
<h3>6.3.3. Slicings<a class="headerlink" href="#slicings" title="Permalink to this headline">¶</a></h3>
A slicing selects a range of items in a sequence object (e.g., a string, tuple
or list). Slicings may be used as expressions or as targets in assignment or
<a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate">del</code></a> statements. The syntax for a slicing:
<pre>
slicing ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate">primary</code></a> &quot;[&quot; <a class="reference internal" href="#grammar-token-slice_list"><code class="xref docutils literal notranslate">slice_list</code></a> &quot;]&quot;
slice_list ::= <a class="reference internal" href="#grammar-token-slice_item"><code class="xref docutils literal notranslate">slice_item</code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-slice_item"><code class="xref docutils literal notranslate">slice_item</code></a>)* [&quot;,&quot;]
slice_item ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> | <a class="reference internal" href="#grammar-token-proper_slice"><code class="xref docutils literal notranslate">proper_slice</code></a>
proper_slice ::= [<a class="reference internal" href="#grammar-token-lower_bound"><code class="xref docutils literal notranslate">lower_bound</code></a>] &quot;:&quot; [<a class="reference internal" href="#grammar-token-upper_bound"><code class="xref docutils literal notranslate">upper_bound</code></a>] [ &quot;:&quot; [<a class="reference internal" href="#grammar-token-stride"><code class="xref docutils literal notranslate">stride</code></a>] ]
lower_bound ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>
upper_bound ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>
stride ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>
</pre>
There is ambiguity in the formal syntax here: anything that looks like an
expression list also looks like a slice list, so any subscription can be
interpreted as a slicing. Rather than further complicating the syntax, this is
disambiguated by defining that in this case the interpretation as a subscription
takes priority over the interpretation as a slicing (this is the case if the
slice list contains no proper slice).
The semantics for a slicing are as follows. The primary is indexed (using the
same <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a> method as
normal subscription) with a key that is constructed from the slice list, as
follows. If the slice list contains at least one comma, the key is a tuple
containing the conversion of the slice items; otherwise, the conversion of the
lone slice item is the key. The conversion of a slice item that is an
expression is that expression. The conversion of a proper slice is a slice
object (see section <a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>) whose <code class="xref py py-attr docutils literal notranslate">start</code>,
<code class="xref py py-attr docutils literal notranslate">stop</code> and <code class="xref py py-attr docutils literal notranslate">step</code> attributes are the values of the
expressions given as lower bound, upper bound and stride, respectively,
substituting <code class="docutils literal notranslate">None</code> for missing expressions.
</div>
<div class="section" id="calls">
<h3>6.3.4. Calls<a class="headerlink" href="#calls" title="Permalink to this headline">¶</a></h3>
A call calls a callable object (e.g., a <a class="reference internal" href="../glossary.html#term-function">function</a>) with a possibly empty
series of <a class="reference internal" href="../glossary.html#term-argument">arguments</a>:
<pre>
call ::= <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate">primary</code></a> &quot;(&quot; [<a class="reference internal" href="#grammar-token-argument_list"><code class="xref docutils literal notranslate">argument_list</code></a> [&quot;,&quot;] | <a class="reference internal" href="#grammar-token-comprehension"><code class="xref docutils literal notranslate">comprehension</code></a>] &quot;)&quot;
argument_list ::= <a class="reference internal" href="#grammar-token-positional_arguments"><code class="xref docutils literal notranslate">positional_arguments</code></a> [&quot;,&quot; <a class="reference internal" href="#grammar-token-starred_and_keywords"><code class="xref docutils literal notranslate">starred_and_keywords</code></a>]
 [&quot;,&quot; <a class="reference internal" href="#grammar-token-keywords_arguments"><code class="xref docutils literal notranslate">keywords_arguments</code></a>]
 | <a class="reference internal" href="#grammar-token-starred_and_keywords"><code class="xref docutils literal notranslate">starred_and_keywords</code></a> [&quot;,&quot; <a class="reference internal" href="#grammar-token-keywords_arguments"><code class="xref docutils literal notranslate">keywords_arguments</code></a>]
 | <a class="reference internal" href="#grammar-token-keywords_arguments"><code class="xref docutils literal notranslate">keywords_arguments</code></a>
positional_arguments ::= [&quot;*&quot;] <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> (&quot;,&quot; [&quot;*&quot;] <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>)*
starred_and_keywords ::= (&quot;*&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> | <a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate">keyword_item</code></a>)
 (&quot;,&quot; &quot;*&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> | &quot;,&quot; <a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate">keyword_item</code></a>)*
keywords_arguments ::= (<a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate">keyword_item</code></a> | &quot;**&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>)
 (&quot;,&quot; <a class="reference internal" href="#grammar-token-keyword_item"><code class="xref docutils literal notranslate">keyword_item</code></a> | &quot;,&quot; &quot;**&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>)*
keyword_item ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate">identifier</code></a> &quot;=&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>
</pre>
An optional trailing comma may be present after the positional and keyword arguments
but does not affect the semantics.
The primary must evaluate to a callable object (user-defined functions, built-in
functions, methods of built-in objects, class objects, methods of class
instances, and all objects having a <a class="reference internal" href="datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate">__call__()</code></a> method are callable). All
argument expressions are evaluated before the call is attempted. Please refer
to section <a class="reference internal" href="compound_stmts.html#function">Function definitions</a> for the syntax of formal <a class="reference internal" href="../glossary.html#term-parameter">parameter</a> lists.
If keyword arguments are present, they are first converted to positional
arguments, as follows. First, a list of unfilled slots is created for the
formal parameters. If there are N positional arguments, they are placed in the
first N slots. Next, for each keyword argument, the identifier is used to
determine the corresponding slot (if the identifier is the same as the first
formal parameter name, the first slot is used, and so on). If the slot is
already filled, a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception is raised. Otherwise, the value of
the argument is placed in the slot, filling it (even if the expression is
<code class="docutils literal notranslate">None</code>, it fills the slot). When all arguments have been processed, the slots
that are still unfilled are filled with the corresponding default value from the
function definition. (Default values are calculated, once, when the function is
defined; thus, a mutable object such as a list or dictionary used as default
value will be shared by all calls that don’t specify an argument value for the
corresponding slot; this should usually be avoided.) If there are any unfilled
slots for which no default value is specified, a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception is
raised. Otherwise, the list of filled slots is used as the argument list for
the call.
<div class="impl-detail compound">
CPython implementation detail: An implementation may provide built-in functions whose positional parameters
do not have names, even if they are ‘named’ for the purpose of documentation,
and which therefore cannot be supplied by keyword. In CPython, this is the
case for functions implemented in C that use <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate">PyArg_ParseTuple()</code></a> to
parse their arguments.
</div>
If there are more positional arguments than there are formal parameter slots, a
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception is raised, unless a formal parameter using the syntax
<code class="docutils literal notranslate">*identifier</code> is present; in this case, that formal parameter receives a tuple
containing the excess positional arguments (or an empty tuple if there were no
excess positional arguments).
If any keyword argument does not correspond to a formal parameter name, a
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception is raised, unless a formal parameter using the syntax
<code class="docutils literal notranslate">**identifier</code> is present; in this case, that formal parameter receives a
dictionary containing the excess keyword arguments (using the keywords as keys
and the argument values as corresponding values), or a (new) empty dictionary if
there were no excess keyword arguments.
If the syntax <code class="docutils literal notranslate">*expression</code> appears in the function call, <code class="docutils literal notranslate">expression</code> must
evaluate to an <a class="reference internal" href="../glossary.html#term-iterable">iterable</a>. Elements from these iterables are
treated as if they were additional positional arguments. For the call
<code class="docutils literal notranslate">f(x1, x2, *y, x3, x4)</code>, if y evaluates to a sequence y1, …, yM,
this is equivalent to a call with M+4 positional arguments x1, x2,
y1, …, yM, x3, x4.
A consequence of this is that although the <code class="docutils literal notranslate">*expression</code> syntax may appear
after explicit keyword arguments, it is processed before the
keyword arguments (and any <code class="docutils literal notranslate">**expression</code> arguments – see below). So:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def f(a, b):
... print(a, b)
...
&gt;&gt;&gt; f(b=1, *(2,))
2 1
&gt;&gt;&gt; f(a=1, *(2,))
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
TypeError: f() got multiple values for keyword argument &#39;a&#39;
&gt;&gt;&gt; f(1, *(2,))
1 2
</pre></div>
</div>
It is unusual for both keyword arguments and the <code class="docutils literal notranslate">*expression</code> syntax to be
used in the same call, so in practice this confusion does not arise.
If the syntax <code class="docutils literal notranslate">**expression</code> appears in the function call, <code class="docutils literal notranslate">expression</code> must
evaluate to a <a class="reference internal" href="../glossary.html#term-mapping">mapping</a>, the contents of which are treated as
additional keyword arguments. If a keyword is already present
(as an explicit keyword argument, or from another unpacking),
a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception is raised.
Formal parameters using the syntax <code class="docutils literal notranslate">*identifier</code> or <code class="docutils literal notranslate">**identifier</code> cannot be
used as positional argument slots or as keyword argument names.
<div class="versionchanged">
Changed in version 3.5: Function calls accept any number of <code class="docutils literal notranslate">*</code> and <code class="docutils literal notranslate">**</code> unpackings,
positional arguments may follow iterable unpackings (<code class="docutils literal notranslate">*</code>),
and keyword arguments may follow dictionary unpackings (<code class="docutils literal notranslate">**</code>).
Originally proposed by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0448">PEP 448</a>.
</div>
A call always returns some value, possibly <code class="docutils literal notranslate">None</code>, unless it raises an
exception. How this value is computed depends on the type of the callable
object.
If it is—
<dl class="docutils">
<dt>a user-defined function:</dt>
<dd>The code block for the function is executed, passing it the argument list. The
first thing the code block will do is bind the formal parameters to the
arguments; this is described in section <a class="reference internal" href="compound_stmts.html#function">Function definitions</a>. When the code block
executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement, this specifies the return value of the
function call.
</dd>
<dt>a built-in function or method:</dt>
<dd>The result is up to the interpreter; see <a class="reference internal" href="../library/functions.html#built-in-funcs">Built-in Functions</a> for the
descriptions of built-in functions and methods.
</dd>
<dt>a class object:</dt>
<dd>A new instance of that class is returned.
</dd>
<dt>a class instance method:</dt>
<dd>The corresponding user-defined function is called, with an argument list that is
one longer than the argument list of the call: the instance becomes the first
argument.
</dd>
<dt>a class instance:</dt>
<dd>The class must define a <a class="reference internal" href="datamodel.html#object.__call__" title="object.__call__"><code class="xref py py-meth docutils literal notranslate">__call__()</code></a> method; the effect is then the same as
if that method was called.
</dd>
</dl>
</div>
</div>
<div class="section" id="await-expression">
<h2>6.4. Await expression<a class="headerlink" href="#await-expression" title="Permalink to this headline">¶</a></h2>
Suspend the execution of <a class="reference internal" href="../glossary.html#term-coroutine">coroutine</a> on an <a class="reference internal" href="../glossary.html#term-awaitable">awaitable</a> object.
Can only be used inside a <a class="reference internal" href="../glossary.html#term-coroutine-function">coroutine function</a>.
<pre>
await_expr ::= &quot;await&quot; <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate">primary</code></a>
</pre>
<div class="versionadded">
New in version 3.5.
</div>
</div>
<div class="section" id="the-power-operator">
<h2>6.5. The power operator<a class="headerlink" href="#the-power-operator" title="Permalink to this headline">¶</a></h2>
The power operator binds more tightly than unary operators on its left; it binds
less tightly than unary operators on its right. The syntax is:
<pre>
power ::= (<a class="reference internal" href="#grammar-token-await_expr"><code class="xref docutils literal notranslate">await_expr</code></a> | <a class="reference internal" href="#grammar-token-primary"><code class="xref docutils literal notranslate">primary</code></a>) [&quot;**&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a>]
</pre>
Thus, in an unparenthesized sequence of power and unary operators, the operators
are evaluated from right to left (this does not constrain the evaluation order
for the operands): <code class="docutils literal notranslate">-1**2</code> results in <code class="docutils literal notranslate">-1</code>.
The power operator has the same semantics as the built-in <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate">pow()</code></a> function,
when called with two arguments: it yields its left argument raised to the power
of its right argument. The numeric arguments are first converted to a common
type, and the result is of that type.
For int operands, the result has the same type as the operands unless the second
argument is negative; in that case, all arguments are converted to float and a
float result is delivered. For example, <code class="docutils literal notranslate">10**2</code> returns <code class="docutils literal notranslate">100</code>, but
<code class="docutils literal notranslate">10**-2</code> returns <code class="docutils literal notranslate">0.01</code>.
Raising <code class="docutils literal notranslate">0.0</code> to a negative power results in a <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate">ZeroDivisionError</code></a>.
Raising a negative number to a fractional power results in a <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate">complex</code></a>
number. (In earlier versions it raised a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>.)
</div>
<div class="section" id="unary-arithmetic-and-bitwise-operations">
<h2>6.6. Unary arithmetic and bitwise operations<a class="headerlink" href="#unary-arithmetic-and-bitwise-operations" title="Permalink to this headline">¶</a></h2>
All unary arithmetic and bitwise operations have the same priority:
<pre>
u_expr ::= <a class="reference internal" href="#grammar-token-power"><code class="xref docutils literal notranslate">power</code></a> | &quot;-&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a> | &quot;+&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a> | &quot;~&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a>
</pre>
The unary <code class="docutils literal notranslate">-</code> (minus) operator yields the negation of its numeric argument.
The unary <code class="docutils literal notranslate">+</code> (plus) operator yields its numeric argument unchanged.
The unary <code class="docutils literal notranslate">~</code> (invert) operator yields the bitwise inversion of its integer
argument. The bitwise inversion of <code class="docutils literal notranslate">x</code> is defined as <code class="docutils literal notranslate">-(x+1)</code>. It only
applies to integral numbers.
In all three cases, if the argument does not have the proper type, a
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception is raised.
</div>
<div class="section" id="binary-arithmetic-operations">
<h2>6.7. Binary arithmetic operations<a class="headerlink" href="#binary-arithmetic-operations" title="Permalink to this headline">¶</a></h2>
The binary arithmetic operations have the conventional priority levels. Note
that some of these operations also apply to certain non-numeric types. Apart
from the power operator, there are only two levels, one for multiplicative
operators and one for additive operators:
<pre>
m_expr ::= <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> &quot;*&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> &quot;&#64;&quot; <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> |
 <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> &quot;//&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a> | <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> &quot;/&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a> |
 <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> &quot;%&quot; <a class="reference internal" href="#grammar-token-u_expr"><code class="xref docutils literal notranslate">u_expr</code></a>
a_expr ::= <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> | <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate">a_expr</code></a> &quot;+&quot; <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a> | <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate">a_expr</code></a> &quot;-&quot; <a class="reference internal" href="#grammar-token-m_expr"><code class="xref docutils literal notranslate">m_expr</code></a>
</pre>
The <code class="docutils literal notranslate">*</code> (multiplication) operator yields the product of its arguments. The
arguments must either both be numbers, or one argument must be an integer and
the other must be a sequence. In the former case, the numbers are converted to a
common type and then multiplied together. In the latter case, sequence
repetition is performed; a negative repetition factor yields an empty sequence.
The <code class="docutils literal notranslate">&#64;</code> (at) operator is intended to be used for matrix multiplication. No
builtin Python types implement this operator.
<div class="versionadded">
New in version 3.5.
</div>
The <code class="docutils literal notranslate">/</code> (division) and <code class="docutils literal notranslate">//</code> (floor division) operators yield the quotient of
their arguments. The numeric arguments are first converted to a common type.
Division of integers yields a float, while floor division of integers results in an
integer; the result is that of mathematical division with the ‘floor’ function
applied to the result. Division by zero raises the <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate">ZeroDivisionError</code></a>
exception.
The <code class="docutils literal notranslate">%</code> (modulo) operator yields the remainder from the division of the first
argument by the second. The numeric arguments are first converted to a common
type. A zero right argument raises the <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate">ZeroDivisionError</code></a> exception. The
arguments may be floating point numbers, e.g., <code class="docutils literal notranslate">3.14%0.7</code> equals <code class="docutils literal notranslate">0.34</code>
(since <code class="docutils literal notranslate">3.14</code> equals <code class="docutils literal notranslate">4*0.7 + 0.34</code>.) The modulo operator always yields a
result with the same sign as its second operand (or zero); the absolute value of
the result is strictly smaller than the absolute value of the second operand
<a class="footnote-reference" href="#id17" id="id9">[1]</a>.
The floor division and modulo operators are connected by the following
identity: <code class="docutils literal notranslate">x == (x//y)*y + (x%y)</code>. Floor division and modulo are also
connected with the built-in function <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate">divmod()</code></a>: <code class="docutils literal notranslate">divmod(x, y) == (x//y,
x%y)</code>. <a class="footnote-reference" href="#id18" id="id10">[2]</a>.
In addition to performing the modulo operation on numbers, the <code class="docutils literal notranslate">%</code> operator is
also overloaded by string objects to perform old-style string formatting (also
known as interpolation). The syntax for string formatting is described in the
Python Library Reference, section <a class="reference internal" href="../library/stdtypes.html#old-string-formatting">printf-style String Formatting</a>.
The floor division operator, the modulo operator, and the <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate">divmod()</code></a>
function are not defined for complex numbers. Instead, convert to a floating
point number using the <a class="reference internal" href="../library/functions.html#abs" title="abs"><code class="xref py py-func docutils literal notranslate">abs()</code></a> function if appropriate.
The <code class="docutils literal notranslate">+</code> (addition) operator yields the sum of its arguments. The arguments
must either both be numbers or both be sequences of the same type. In the
former case, the numbers are converted to a common type and then added together.
In the latter case, the sequences are concatenated.
The <code class="docutils literal notranslate">-</code> (subtraction) operator yields the difference of its arguments. The
numeric arguments are first converted to a common type.
</div>
<div class="section" id="shifting-operations">
<h2>6.8. Shifting operations<a class="headerlink" href="#shifting-operations" title="Permalink to this headline">¶</a></h2>
The shifting operations have lower priority than the arithmetic operations:
<pre>
shift_expr ::= <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate">a_expr</code></a> | <a class="reference internal" href="#grammar-token-shift_expr"><code class="xref docutils literal notranslate">shift_expr</code></a> (&quot;&lt;&lt;&quot; | &quot;&gt;&gt;&quot;) <a class="reference internal" href="#grammar-token-a_expr"><code class="xref docutils literal notranslate">a_expr</code></a>
</pre>
These operators accept integers as arguments. They shift the first argument to
the left or right by the number of bits given by the second argument.
A right shift by n bits is defined as floor division by <code class="docutils literal notranslate">pow(2,n)</code>. A left
shift by n bits is defined as multiplication with <code class="docutils literal notranslate">pow(2,n)</code>.
<div class="admonition note">
Note
In the current implementation, the right-hand operand is required
to be at most <a class="reference internal" href="../library/sys.html#sys.maxsize" title="sys.maxsize"><code class="xref py py-attr docutils literal notranslate">sys.maxsize</code></a>. If the right-hand operand is larger than
<a class="reference internal" href="../library/sys.html#sys.maxsize" title="sys.maxsize"><code class="xref py py-attr docutils literal notranslate">sys.maxsize</code></a> an <a class="reference internal" href="../library/exceptions.html#OverflowError" title="OverflowError"><code class="xref py py-exc docutils literal notranslate">OverflowError</code></a> exception is raised.
</div>
</div>
<div class="section" id="binary-bitwise-operations">
<h2>6.9. Binary bitwise operations<a class="headerlink" href="#binary-bitwise-operations" title="Permalink to this headline">¶</a></h2>
Each of the three bitwise operations has a different priority level:
<pre>
and_expr ::= <a class="reference internal" href="#grammar-token-shift_expr"><code class="xref docutils literal notranslate">shift_expr</code></a> | <a class="reference internal" href="#grammar-token-and_expr"><code class="xref docutils literal notranslate">and_expr</code></a> &quot;&amp;&quot; <a class="reference internal" href="#grammar-token-shift_expr"><code class="xref docutils literal notranslate">shift_expr</code></a>
xor_expr ::= <a class="reference internal" href="#grammar-token-and_expr"><code class="xref docutils literal notranslate">and_expr</code></a> | <a class="reference internal" href="#grammar-token-xor_expr"><code class="xref docutils literal notranslate">xor_expr</code></a> &quot;^&quot; <a class="reference internal" href="#grammar-token-and_expr"><code class="xref docutils literal notranslate">and_expr</code></a>
or_expr ::= <a class="reference internal" href="#grammar-token-xor_expr"><code class="xref docutils literal notranslate">xor_expr</code></a> | <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate">or_expr</code></a> &quot;|&quot; <a class="reference internal" href="#grammar-token-xor_expr"><code class="xref docutils literal notranslate">xor_expr</code></a>
</pre>
The <code class="docutils literal notranslate">&amp;</code> operator yields the bitwise AND of its arguments, which must be
integers.
The <code class="docutils literal notranslate">^</code> operator yields the bitwise XOR (exclusive OR) of its arguments, which
must be integers.
The <code class="docutils literal notranslate">|</code> operator yields the bitwise (inclusive) OR of its arguments, which
must be integers.
</div>
<div class="section" id="comparisons">
<h2>6.10. Comparisons<a class="headerlink" href="#comparisons" title="Permalink to this headline">¶</a></h2>
Unlike C, all comparison operations in Python have the same priority, which is
lower than that of any arithmetic, shifting or bitwise operation. Also unlike
C, expressions like <code class="docutils literal notranslate">a &lt; b &lt; c</code> have the interpretation that is conventional
in mathematics:
<pre>
comparison ::= <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate">or_expr</code></a> (<a class="reference internal" href="#grammar-token-comp_operator"><code class="xref docutils literal notranslate">comp_operator</code></a> <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate">or_expr</code></a>)*
comp_operator ::= &quot;&lt;&quot; | &quot;&gt;&quot; | &quot;==&quot; | &quot;&gt;=&quot; | &quot;&lt;=&quot; | &quot;!=&quot;
 | &quot;is&quot; [&quot;not&quot;] | [&quot;not&quot;] &quot;in&quot;
</pre>
Comparisons yield boolean values: <code class="docutils literal notranslate">True</code> or <code class="docutils literal notranslate">False</code>.
Comparisons can be chained arbitrarily, e.g., <code class="docutils literal notranslate">x &lt; y &lt;= z</code> is equivalent to
<code class="docutils literal notranslate">x &lt; y and y &lt;= z</code>, except that <code class="docutils literal notranslate">y</code> is evaluated only once (but in both
cases <code class="docutils literal notranslate">z</code> is not evaluated at all when <code class="docutils literal notranslate">x &lt; y</code> is found to be false).
Formally, if a, b, c, …, y, z are expressions and op1, op2, …,
opN are comparison operators, then <code class="docutils literal notranslate">a op1 b op2 c ... y opN z</code> is equivalent
to <code class="docutils literal notranslate">a op1 b and b op2 c and ... y opN z</code>, except that each expression is
evaluated at most once.
Note that <code class="docutils literal notranslate">a op1 b op2 c</code> doesn’t imply any kind of comparison between a and
c, so that, e.g., <code class="docutils literal notranslate">x &lt; y &gt; z</code> is perfectly legal (though perhaps not
pretty).
<div class="section" id="value-comparisons">
<h3>6.10.1. Value comparisons<a class="headerlink" href="#value-comparisons" title="Permalink to this headline">¶</a></h3>
The operators <code class="docutils literal notranslate">&lt;</code>, <code class="docutils literal notranslate">&gt;</code>, <code class="docutils literal notranslate">==</code>, <code class="docutils literal notranslate">&gt;=</code>, <code class="docutils literal notranslate">&lt;=</code>, and <code class="docutils literal notranslate">!=</code> compare the
values of two objects. The objects do not need to have the same type.
Chapter <a class="reference internal" href="datamodel.html#objects">Objects, values and types</a> states that objects have a value (in addition to type
and identity). The value of an object is a rather abstract notion in Python:
For example, there is no canonical access method for an object’s value. Also,
there is no requirement that the value of an object should be constructed in a
particular way, e.g. comprised of all its data attributes. Comparison operators
implement a particular notion of what the value of an object is. One can think
of them as defining the value of an object indirectly, by means of their
comparison implementation.
Because all types are (direct or indirect) subtypes of <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>, they
inherit the default comparison behavior from <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>. Types can
customize their comparison behavior by implementing
rich comparison methods like <a class="reference internal" href="datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate">__lt__()</code></a>, described in
<a class="reference internal" href="datamodel.html#customization">Basic customization</a>.
The default behavior for equality comparison (<code class="docutils literal notranslate">==</code> and <code class="docutils literal notranslate">!=</code>) is based on
the identity of the objects. Hence, equality comparison of instances with the
same identity results in equality, and equality comparison of instances with
different identities results in inequality. A motivation for this default
behavior is the desire that all objects should be reflexive (i.e. <code class="docutils literal notranslate">x is y</code>
implies <code class="docutils literal notranslate">x == y</code>).
A default order comparison (<code class="docutils literal notranslate">&lt;</code>, <code class="docutils literal notranslate">&gt;</code>, <code class="docutils literal notranslate">&lt;=</code>, and <code class="docutils literal notranslate">&gt;=</code>) is not provided;
an attempt raises <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>. A motivation for this default behavior is
the lack of a similar invariant as for equality.
The behavior of the default equality comparison, that instances with different
identities are always unequal, may be in contrast to what types will need that
have a sensible definition of object value and value-based equality. Such
types will need to customize their comparison behavior, and in fact, a number
of built-in types have done that.
The following list describes the comparison behavior of the most important
built-in types.
<ul>
<li>Numbers of built-in numeric types (<a class="reference internal" href="../library/stdtypes.html#typesnumeric">Numeric Types — int, float, complex</a>) and of the standard
library types <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate">fractions.Fraction</code></a> and <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">decimal.Decimal</code></a> can be
compared within and across their types, with the restriction that complex
numbers do not support order comparison. Within the limits of the types
involved, they compare mathematically (algorithmically) correct without loss
of precision.
The not-a-number values <code class="xref py py-const docutils literal notranslate">float('NaN')</code> and <code class="xref py py-const docutils literal notranslate">Decimal('NaN')</code>
are special. They are identical to themselves (<code class="docutils literal notranslate">x is x</code> is true) but
are not equal to themselves (<code class="docutils literal notranslate">x == x</code> is false). Additionally,
comparing any number to a not-a-number value
will return <code class="docutils literal notranslate">False</code>. For example, both <code class="docutils literal notranslate">3 &lt; float('NaN')</code> and
<code class="docutils literal notranslate">float('NaN') &lt; 3</code> will return <code class="docutils literal notranslate">False</code>.
</li>
<li>Binary sequences (instances of <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> or <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a>) can be
compared within and across their types. They compare lexicographically using
the numeric values of their elements.
</li>
<li>Strings (instances of <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>) compare lexicographically using the
numerical Unicode code points (the result of the built-in function
<a class="reference internal" href="../library/functions.html#ord" title="ord"><code class="xref py py-func docutils literal notranslate">ord()</code></a>) of their characters. <a class="footnote-reference" href="#id19" id="id12">[3]</a>
Strings and binary sequences cannot be directly compared.
</li>
<li>Sequences (instances of <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a>, <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a>, or <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-class docutils literal notranslate">range</code></a>) can
be compared only within each of their types, with the restriction that ranges
do not support order comparison. Equality comparison across these types
results in inequality, and ordering comparison across these types raises
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.
Sequences compare lexicographically using comparison of corresponding
elements, whereby reflexivity of the elements is enforced.
In enforcing reflexivity of elements, the comparison of collections assumes
that for a collection element <code class="docutils literal notranslate">x</code>, <code class="docutils literal notranslate">x == x</code> is always true. Based on
that assumption, element identity is compared first, and element comparison
is performed only for distinct elements. This approach yields the same
result as a strict element comparison would, if the compared elements are
reflexive. For non-reflexive elements, the result is different than for
strict element comparison, and may be surprising: The non-reflexive
not-a-number values for example result in the following comparison behavior
when used in a list:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; nan = float(&#39;NaN&#39;)
&gt;&gt;&gt; nan is nan
True
&gt;&gt;&gt; nan == nan
False &lt;-- the defined non-reflexive behavior of NaN
&gt;&gt;&gt; [nan] == [nan]
True &lt;-- list enforces reflexivity and tests identity first
</pre></div>
</div>
Lexicographical comparison between built-in collections works as follows:
<ul class="simple">
<li>For two collections to compare equal, they must be of the same type, have
the same length, and each pair of corresponding elements must compare
equal (for example, <code class="docutils literal notranslate">[1,2] == (1,2)</code> is false because the type is not the
same).</li>
<li>Collections that support order comparison are ordered the same as their
first unequal elements (for example, <code class="docutils literal notranslate">[1,2,x] &lt;= [1,2,y]</code> has the same
value as <code class="docutils literal notranslate">x &lt;= y</code>). If a corresponding element does not exist, the
shorter collection is ordered first (for example, <code class="docutils literal notranslate">[1,2] &lt; [1,2,3]</code> is
true).</li>
</ul>
</li>
<li>Mappings (instances of <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>) compare equal if and only if they have
equal <cite>(key, value)</cite> pairs. Equality comparison of the keys and values
enforces reflexivity.
Order comparisons (<code class="docutils literal notranslate">&lt;</code>, <code class="docutils literal notranslate">&gt;</code>, <code class="docutils literal notranslate">&lt;=</code>, and <code class="docutils literal notranslate">&gt;=</code>) raise <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.
</li>
<li>Sets (instances of <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a> or <a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate">frozenset</code></a>) can be compared within
and across their types.
They define order
comparison operators to mean subset and superset tests. Those relations do
not define total orderings (for example, the two sets <code class="docutils literal notranslate">{1,2}</code> and <code class="docutils literal notranslate">{2,3}</code>
are not equal, nor subsets of one another, nor supersets of one
another). Accordingly, sets are not appropriate arguments for functions
which depend on total ordering (for example, <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate">min()</code></a>, <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate">max()</code></a>, and
<a class="reference internal" href="../library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate">sorted()</code></a> produce undefined results given a list of sets as inputs).
Comparison of sets enforces reflexivity of its elements.
</li>
<li>Most other built-in types have no comparison methods implemented, so they
inherit the default comparison behavior.
</li>
</ul>
User-defined classes that customize their comparison behavior should follow
some consistency rules, if possible:
<ul>
<li>Equality comparison should be reflexive.
In other words, identical objects should compare equal:
<blockquote>
<div><code class="docutils literal notranslate">x is y</code> implies <code class="docutils literal notranslate">x == y</code>
</div></blockquote>
</li>
<li>Comparison should be symmetric.
In other words, the following expressions should have the same result:
<blockquote>
<div><code class="docutils literal notranslate">x == y</code> and <code class="docutils literal notranslate">y == x</code>
<code class="docutils literal notranslate">x != y</code> and <code class="docutils literal notranslate">y != x</code>
<code class="docutils literal notranslate">x &lt; y</code> and <code class="docutils literal notranslate">y &gt; x</code>
<code class="docutils literal notranslate">x &lt;= y</code> and <code class="docutils literal notranslate">y &gt;= x</code>
</div></blockquote>
</li>
<li>Comparison should be transitive.
The following (non-exhaustive) examples illustrate that:
<blockquote>
<div><code class="docutils literal notranslate">x &gt; y and y &gt; z</code> implies <code class="docutils literal notranslate">x &gt; z</code>
<code class="docutils literal notranslate">x &lt; y and y &lt;= z</code> implies <code class="docutils literal notranslate">x &lt; z</code>
</div></blockquote>
</li>
<li>Inverse comparison should result in the boolean negation.
In other words, the following expressions should have the same result:
<blockquote>
<div><code class="docutils literal notranslate">x == y</code> and <code class="docutils literal notranslate">not x != y</code>
<code class="docutils literal notranslate">x &lt; y</code> and <code class="docutils literal notranslate">not x &gt;= y</code> (for total ordering)
<code class="docutils literal notranslate">x &gt; y</code> and <code class="docutils literal notranslate">not x &lt;= y</code> (for total ordering)
</div></blockquote>
The last two expressions apply to totally ordered collections (e.g. to
sequences, but not to sets or mappings). See also the
<a class="reference internal" href="../library/functools.html#functools.total_ordering" title="functools.total_ordering"><code class="xref py py-func docutils literal notranslate">total_ordering()</code></a> decorator.
</li>
<li>The <a class="reference internal" href="../library/functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate">hash()</code></a> result should be consistent with equality.
Objects that are equal should either have the same hash value,
or be marked as unhashable.
</li>
</ul>
Python does not enforce these consistency rules. In fact, the not-a-number
values are an example for not following these rules.
</div>
<div class="section" id="membership-test-operations">
<h3>6.10.2. Membership test operations<a class="headerlink" href="#membership-test-operations" title="Permalink to this headline">¶</a></h3>
The operators <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> and <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate">not in</code></a> test for membership. <code class="docutils literal notranslate">x in
s</code> evaluates to <code class="docutils literal notranslate">True</code> if x is a member of s, and <code class="docutils literal notranslate">False</code> otherwise.
<code class="docutils literal notranslate">x not in s</code> returns the negation of <code class="docutils literal notranslate">x in s</code>. All built-in sequences and
set types support this as well as dictionary, for which <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> tests
whether the dictionary has a given key. For container types such as list, tuple,
set, frozenset, dict, or collections.deque, the expression <code class="docutils literal notranslate">x in y</code> is equivalent
to <code class="docutils literal notranslate">any(x is e or x == e for e in y)</code>.
For the string and bytes types, <code class="docutils literal notranslate">x in y</code> is <code class="docutils literal notranslate">True</code> if and only if x is a
substring of y. An equivalent test is <code class="docutils literal notranslate">y.find(x) != -1</code>. Empty strings are
always considered to be a substring of any other string, so <code class="docutils literal notranslate">&quot;&quot; in &quot;abc&quot;</code> will
return <code class="docutils literal notranslate">True</code>.
For user-defined classes which define the <a class="reference internal" href="datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate">__contains__()</code></a> method, <code class="docutils literal notranslate">x in
y</code> returns <code class="docutils literal notranslate">True</code> if <code class="docutils literal notranslate">y.__contains__(x)</code> returns a true value, and
<code class="docutils literal notranslate">False</code> otherwise.
For user-defined classes which do not define <a class="reference internal" href="datamodel.html#object.__contains__" title="object.__contains__"><code class="xref py py-meth docutils literal notranslate">__contains__()</code></a> but do define
<a class="reference internal" href="datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal notranslate">__iter__()</code></a>, <code class="docutils literal notranslate">x in y</code> is <code class="docutils literal notranslate">True</code> if some value <code class="docutils literal notranslate">z</code> with <code class="docutils literal notranslate">x == z</code> is
produced while iterating over <code class="docutils literal notranslate">y</code>. If an exception is raised during the
iteration, it is as if <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> raised that exception.
Lastly, the old-style iteration protocol is tried: if a class defines
<a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a>, <code class="docutils literal notranslate">x in y</code> is <code class="docutils literal notranslate">True</code> if and only if there is a non-negative
integer index i such that <code class="docutils literal notranslate">x == y[i]</code>, and all lower integer indices do not
raise <a class="reference internal" href="../library/exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate">IndexError</code></a> exception. (If any other exception is raised, it is as
if <a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> raised that exception).
The operator <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate">not in</code></a> is defined to have the inverse true value of
<a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a>.
</div>
<div class="section" id="is-not">
<h3>6.10.3. Identity comparisons<a class="headerlink" href="#is-not" title="Permalink to this headline">¶</a></h3>
The operators <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate">is</code></a> and <a class="reference internal" href="#is-not"><code class="xref std std-keyword docutils literal notranslate">is not</code></a> test for object identity: <code class="docutils literal notranslate">x
is y</code> is true if and only if x and y are the same object. Object identity
is determined using the <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-meth docutils literal notranslate">id()</code></a> function. <code class="docutils literal notranslate">x is not y</code> yields the inverse
truth value. <a class="footnote-reference" href="#id20" id="id13">[4]</a>
</div>
</div>
<div class="section" id="boolean-operations">
<h2>6.11. Boolean operations<a class="headerlink" href="#boolean-operations" title="Permalink to this headline">¶</a></h2>
<pre id="index-73">
or_test ::= <a class="reference internal" href="#grammar-token-and_test"><code class="xref docutils literal notranslate">and_test</code></a> | <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate">or_test</code></a> &quot;or&quot; <a class="reference internal" href="#grammar-token-and_test"><code class="xref docutils literal notranslate">and_test</code></a>
and_test ::= <a class="reference internal" href="#grammar-token-not_test"><code class="xref docutils literal notranslate">not_test</code></a> | <a class="reference internal" href="#grammar-token-and_test"><code class="xref docutils literal notranslate">and_test</code></a> &quot;and&quot; <a class="reference internal" href="#grammar-token-not_test"><code class="xref docutils literal notranslate">not_test</code></a>
not_test ::= <a class="reference internal" href="#grammar-token-comparison"><code class="xref docutils literal notranslate">comparison</code></a> | &quot;not&quot; <a class="reference internal" href="#grammar-token-not_test"><code class="xref docutils literal notranslate">not_test</code></a>
</pre>
In the context of Boolean operations, and also when expressions are used by
control flow statements, the following values are interpreted as false:
<code class="docutils literal notranslate">False</code>, <code class="docutils literal notranslate">None</code>, numeric zero of all types, and empty strings and containers
(including strings, tuples, lists, dictionaries, sets and frozensets). All
other values are interpreted as true. User-defined objects can customize their
truth value by providing a <a class="reference internal" href="datamodel.html#object.__bool__" title="object.__bool__"><code class="xref py py-meth docutils literal notranslate">__bool__()</code></a> method.
The operator <a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate">not</code></a> yields <code class="docutils literal notranslate">True</code> if its argument is false, <code class="docutils literal notranslate">False</code>
otherwise.
The expression <code class="docutils literal notranslate">x and y</code> first evaluates x; if x is false, its value is
returned; otherwise, y is evaluated and the resulting value is returned.
The expression <code class="docutils literal notranslate">x or y</code> first evaluates x; if x is true, its value is
returned; otherwise, y is evaluated and the resulting value is returned.
(Note that neither <a class="reference internal" href="#and"><code class="xref std std-keyword docutils literal notranslate">and</code></a> nor <a class="reference internal" href="#or"><code class="xref std std-keyword docutils literal notranslate">or</code></a> restrict the value and type
they return to <code class="docutils literal notranslate">False</code> and <code class="docutils literal notranslate">True</code>, but rather return the last evaluated
argument. This is sometimes useful, e.g., if <code class="docutils literal notranslate">s</code> is a string that should be
replaced by a default value if it is empty, the expression <code class="docutils literal notranslate">s or 'foo'</code> yields
the desired value. Because <a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate">not</code></a> has to create a new value, it
returns a boolean value regardless of the type of its argument
(for example, <code class="docutils literal notranslate">not 'foo'</code> produces <code class="docutils literal notranslate">False</code> rather than <code class="docutils literal notranslate">''</code>.)
</div>
<div class="section" id="conditional-expressions">
<h2>6.12. Conditional expressions<a class="headerlink" href="#conditional-expressions" title="Permalink to this headline">¶</a></h2>
<pre id="index-77">
conditional_expression ::= <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate">or_test</code></a> [&quot;if&quot; <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate">or_test</code></a> &quot;else&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>]
expression ::= <a class="reference internal" href="#grammar-token-conditional_expression"><code class="xref docutils literal notranslate">conditional_expression</code></a> | <a class="reference internal" href="#grammar-token-lambda_expr"><code class="xref docutils literal notranslate">lambda_expr</code></a>
expression_nocond ::= <a class="reference internal" href="#grammar-token-or_test"><code class="xref docutils literal notranslate">or_test</code></a> | <a class="reference internal" href="#grammar-token-lambda_expr_nocond"><code class="xref docutils literal notranslate">lambda_expr_nocond</code></a>
</pre>
Conditional expressions (sometimes called a “ternary operator”) have the lowest
priority of all Python operations.
The expression <code class="docutils literal notranslate">x if C else y</code> first evaluates the condition, C rather than x.
If C is true, x is evaluated and its value is returned; otherwise, y is
evaluated and its value is returned.
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0308">PEP 308</a> for more details about conditional expressions.
</div>
<div class="section" id="lambda">
<h2>6.13. Lambdas<a class="headerlink" href="#lambda" title="Permalink to this headline">¶</a></h2>
<pre id="index-79">
lambda_expr ::= &quot;lambda&quot; [<a class="reference internal" href="compound_stmts.html#grammar-token-parameter_list"><code class="xref docutils literal notranslate">parameter_list</code></a>]: <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>
lambda_expr_nocond ::= &quot;lambda&quot; [<a class="reference internal" href="compound_stmts.html#grammar-token-parameter_list"><code class="xref docutils literal notranslate">parameter_list</code></a>]: <a class="reference internal" href="#grammar-token-expression_nocond"><code class="xref docutils literal notranslate">expression_nocond</code></a>
</pre>
Lambda expressions (sometimes called lambda forms) are used to create anonymous
functions. The expression <code class="docutils literal notranslate">lambda parameters: expression</code> yields a function
object. The unnamed object behaves like a function object defined with:
<div class="highlight-none notranslate"><div class="highlight"><pre>def &lt;lambda&gt;(parameters):
 return expression
</pre></div>
</div>
See section <a class="reference internal" href="compound_stmts.html#function">Function definitions</a> for the syntax of parameter lists. Note that
functions created with lambda expressions cannot contain statements or
annotations.
</div>
<div class="section" id="expression-lists">
<h2>6.14. Expression lists<a class="headerlink" href="#expression-lists" title="Permalink to this headline">¶</a></h2>
<pre id="index-80">
expression_list ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a>)* [&quot;,&quot;]
starred_list ::= <a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate">starred_item</code></a> (&quot;,&quot; <a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate">starred_item</code></a>)* [&quot;,&quot;]
starred_expression ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> | (<a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate">starred_item</code></a> &quot;,&quot;)* [<a class="reference internal" href="#grammar-token-starred_item"><code class="xref docutils literal notranslate">starred_item</code></a>]
starred_item ::= <a class="reference internal" href="#grammar-token-expression"><code class="xref docutils literal notranslate">expression</code></a> | &quot;*&quot; <a class="reference internal" href="#grammar-token-or_expr"><code class="xref docutils literal notranslate">or_expr</code></a>
</pre>
Except when part of a list or set display, an expression list
containing at least one comma yields a tuple. The length of
the tuple is the number of expressions in the list. The expressions are
evaluated from left to right.
An asterisk <code class="docutils literal notranslate">*</code> denotes iterable unpacking. Its operand must be
an <a class="reference internal" href="../glossary.html#term-iterable">iterable</a>. The iterable is expanded into a sequence of items,
which are included in the new tuple, list, or set, at the site of
the unpacking.
<div class="versionadded">
New in version 3.5: Iterable unpacking in expression lists, originally proposed by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0448">PEP 448</a>.
</div>
The trailing comma is required only to create a single tuple (a.k.a. a
singleton); it is optional in all other cases. A single expression without a
trailing comma doesn’t create a tuple, but rather yields the value of that
expression. (To create an empty tuple, use an empty pair of parentheses:
<code class="docutils literal notranslate">()</code>.)
</div>
<div class="section" id="evaluation-order">
<h2>6.15. Evaluation order<a class="headerlink" href="#evaluation-order" title="Permalink to this headline">¶</a></h2>
Python evaluates expressions from left to right. Notice that while evaluating
an assignment, the right-hand side is evaluated before the left-hand side.
In the following lines, expressions will be evaluated in the arithmetic order of
their suffixes:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>expr1, expr2, expr3, expr4
(expr1, expr2, expr3, expr4)
{expr1: expr2, expr3: expr4}
expr1 + expr2 * (expr3 - expr4)
expr1(expr2, expr3, *expr4, **expr5)
expr3, expr4 = expr1, expr2
</pre></div>
</div>
</div>
<div class="section" id="operator-precedence">
<h2>6.16. Operator precedence<a class="headerlink" href="#operator-precedence" title="Permalink to this headline">¶</a></h2>
The following table summarizes the operator precedence in Python, from lowest
precedence (least binding) to highest precedence (most binding). Operators in
the same box have the same precedence. Unless the syntax is explicitly given,
operators are binary. Operators in the same box group left to right (except for
exponentiation, which groups from right to left).
Note that comparisons, membership tests, and identity tests, all have the same
precedence and have a left-to-right chaining feature as described in the
<a class="reference internal" href="#comparisons">Comparisons</a> section.
<table border="1" class="docutils">
<colgroup>
<col width="56%" />
<col width="44%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Operator</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><a class="reference internal" href="#lambda"><code class="xref std std-keyword docutils literal notranslate">lambda</code></a></td>
<td>Lambda expression</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="compound_stmts.html#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> – <a class="reference internal" href="compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a></td>
<td>Conditional expression</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#or"><code class="xref std std-keyword docutils literal notranslate">or</code></a></td>
<td>Boolean OR</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#and"><code class="xref std std-keyword docutils literal notranslate">and</code></a></td>
<td>Boolean AND</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#not"><code class="xref std std-keyword docutils literal notranslate">not</code></a> <code class="docutils literal notranslate">x</code></td>
<td>Boolean NOT</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a>, <a class="reference internal" href="#not-in"><code class="xref std std-keyword docutils literal notranslate">not in</code></a>,
<a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate">is</code></a>, <a class="reference internal" href="#is-not"><code class="xref std std-keyword docutils literal notranslate">is not</code></a>, <code class="docutils literal notranslate">&lt;</code>,
<code class="docutils literal notranslate">&lt;=</code>, <code class="docutils literal notranslate">&gt;</code>, <code class="docutils literal notranslate">&gt;=</code>, <code class="docutils literal notranslate">!=</code>, <code class="docutils literal notranslate">==</code></td>
<td>Comparisons, including membership
tests and identity tests</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">|</code></td>
<td>Bitwise OR</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">^</code></td>
<td>Bitwise XOR</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">&amp;</code></td>
<td>Bitwise AND</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">&lt;&lt;</code>, <code class="docutils literal notranslate">&gt;&gt;</code></td>
<td>Shifts</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">+</code>, <code class="docutils literal notranslate">-</code></td>
<td>Addition and subtraction</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">*</code>, <code class="docutils literal notranslate">&#64;</code>, <code class="docutils literal notranslate">/</code>, <code class="docutils literal notranslate">//</code>, <code class="docutils literal notranslate">%</code></td>
<td>Multiplication, matrix
multiplication, division, floor
division, remainder <a class="footnote-reference" href="#id21" id="id15">[5]</a></td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">+x</code>, <code class="docutils literal notranslate">-x</code>, <code class="docutils literal notranslate">~x</code></td>
<td>Positive, negative, bitwise NOT</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">**</code></td>
<td>Exponentiation <a class="footnote-reference" href="#id22" id="id16">[6]</a></td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">await</code> <code class="docutils literal notranslate">x</code></td>
<td>Await expression</td>
</tr>
<tr class="row-odd"><td><code class="docutils literal notranslate">x[index]</code>, <code class="docutils literal notranslate">x[index:index]</code>,
<code class="docutils literal notranslate">x(arguments...)</code>, <code class="docutils literal notranslate">x.attribute</code></td>
<td>Subscription, slicing,
call, attribute reference</td>
</tr>
<tr class="row-even"><td><code class="docutils literal notranslate">(expressions...)</code>,
<code class="docutils literal notranslate">[expressions...]</code>,
<code class="docutils literal notranslate">{key: value...}</code>,
<code class="docutils literal notranslate">{expressions...}</code></td>
<td>Binding or tuple display,
list display,
dictionary display,
set display</td>
</tr>
</tbody>
</table>
Footnotes
<table class="docutils footnote" frame="void" id="id17" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id9">[1]</a></td><td>While <code class="docutils literal notranslate">abs(x%y) &lt; abs(y)</code> is true mathematically, for floats it may not be
true numerically due to roundoff. For example, and assuming a platform on which
a Python float is an IEEE 754 double-precision number, in order that <code class="docutils literal notranslate">-1e-100 %
1e100</code> have the same sign as <code class="docutils literal notranslate">1e100</code>, the computed result is <code class="docutils literal notranslate">-1e-100 +
1e100</code>, which is numerically exactly equal to <code class="docutils literal notranslate">1e100</code>. The function
<a class="reference internal" href="../library/math.html#math.fmod" title="math.fmod"><code class="xref py py-func docutils literal notranslate">math.fmod()</code></a> returns a result whose sign matches the sign of the
first argument instead, and so returns <code class="docutils literal notranslate">-1e-100</code> in this case. Which approach
is more appropriate depends on the application.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id18" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id10">[2]</a></td><td>If x is very close to an exact integer multiple of y, it’s possible for
<code class="docutils literal notranslate">x//y</code> to be one larger than <code class="docutils literal notranslate">(x-x%y)//y</code> due to rounding. In such
cases, Python returns the latter result, in order to preserve that
<code class="docutils literal notranslate">divmod(x,y)[0] * y + x % y</code> be very close to <code class="docutils literal notranslate">x</code>.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id19" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id12">[3]</a></td><td>The Unicode standard distinguishes between code points
(e.g. U+0041) and abstract characters (e.g. “LATIN CAPITAL LETTER A”).
While most abstract characters in Unicode are only represented using one
code point, there is a number of abstract characters that can in addition be
represented using a sequence of more than one code point. For example, the
abstract character “LATIN CAPITAL LETTER C WITH CEDILLA” can be represented
as a single precomposed character at code position U+00C7, or as a
sequence of a base character at code position U+0043 (LATIN CAPITAL
LETTER C), followed by a combining character at code position U+0327
(COMBINING CEDILLA).
The comparison operators on strings compare at the level of Unicode code
points. This may be counter-intuitive to humans. For example,
<code class="docutils literal notranslate">&quot;\u00C7&quot; == &quot;\u0043\u0327&quot;</code> is <code class="docutils literal notranslate">False</code>, even though both strings
represent the same abstract character “LATIN CAPITAL LETTER C WITH CEDILLA”.
To compare strings at the level of abstract characters (that is, in a way
intuitive to humans), use <a class="reference internal" href="../library/unicodedata.html#unicodedata.normalize" title="unicodedata.normalize"><code class="xref py py-func docutils literal notranslate">unicodedata.normalize()</code></a>.
</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id20" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id13">[4]</a></td><td>Due to automatic garbage-collection, free lists, and the dynamic nature of
descriptors, you may notice seemingly unusual behaviour in certain uses of
the <a class="reference internal" href="#is"><code class="xref std std-keyword docutils literal notranslate">is</code></a> operator, like those involving comparisons between instance
methods, or constants. Check their documentation for more info.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id21" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id15">[5]</a></td><td>The <code class="docutils literal notranslate">%</code> operator is also used for string formatting; the same
precedence applies.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id22" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id16">[6]</a></td><td>The power operator <code class="docutils literal notranslate">**</code> binds less tightly than an arithmetic or
bitwise unary operator on its right, that is, <code class="docutils literal notranslate">2**-1</code> is <code class="docutils literal notranslate">0.5</code>.</td></tr>
</tbody>
</table>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">6. Expressions</a><ul>
<li><a class="reference internal" href="#arithmetic-conversions">6.1. Arithmetic conversions</a></li>
<li><a class="reference internal" href="#atoms">6.2. Atoms</a><ul>
<li><a class="reference internal" href="#atom-identifiers">6.2.1. Identifiers (Names)</a></li>
<li><a class="reference internal" href="#literals">6.2.2. Literals</a></li>
<li><a class="reference internal" href="#parenthesized-forms">6.2.3. Parenthesized forms</a></li>
<li><a class="reference internal" href="#displays-for-lists-sets-and-dictionaries">6.2.4. Displays for lists, sets and dictionaries</a></li>
<li><a class="reference internal" href="#list-displays">6.2.5. List displays</a></li>
<li><a class="reference internal" href="#set-displays">6.2.6. Set displays</a></li>
<li><a class="reference internal" href="#dictionary-displays">6.2.7. Dictionary displays</a></li>
<li><a class="reference internal" href="#generator-expressions">6.2.8. Generator expressions</a></li>
<li><a class="reference internal" href="#yield-expressions">6.2.9. Yield expressions</a><ul>
<li><a class="reference internal" href="#generator-iterator-methods">6.2.9.1. Generator-iterator methods</a></li>
<li><a class="reference internal" href="#examples">6.2.9.2. Examples</a></li>
<li><a class="reference internal" href="#asynchronous-generator-functions">6.2.9.3. Asynchronous generator functions</a></li>
<li><a class="reference internal" href="#asynchronous-generator-iterator-methods">6.2.9.4. Asynchronous generator-iterator methods</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#primaries">6.3. Primaries</a><ul>
<li><a class="reference internal" href="#attribute-references">6.3.1. Attribute references</a></li>
<li><a class="reference internal" href="#subscriptions">6.3.2. Subscriptions</a></li>
<li><a class="reference internal" href="#slicings">6.3.3. Slicings</a></li>
<li><a class="reference internal" href="#calls">6.3.4. Calls</a></li>
</ul>
</li>
<li><a class="reference internal" href="#await-expression">6.4. Await expression</a></li>
<li><a class="reference internal" href="#the-power-operator">6.5. The power operator</a></li>
<li><a class="reference internal" href="#unary-arithmetic-and-bitwise-operations">6.6. Unary arithmetic and bitwise operations</a></li>
<li><a class="reference internal" href="#binary-arithmetic-operations">6.7. Binary arithmetic operations</a></li>
<li><a class="reference internal" href="#shifting-operations">6.8. Shifting operations</a></li>
<li><a class="reference internal" href="#binary-bitwise-operations">6.9. Binary bitwise operations</a></li>
<li><a class="reference internal" href="#comparisons">6.10. Comparisons</a><ul>
<li><a class="reference internal" href="#value-comparisons">6.10.1. Value comparisons</a></li>
<li><a class="reference internal" href="#membership-test-operations">6.10.2. Membership test operations</a></li>
<li><a class="reference internal" href="#is-not">6.10.3. Identity comparisons</a></li>
</ul>
</li>
<li><a class="reference internal" href="#boolean-operations">6.11. Boolean operations</a></li>
<li><a class="reference internal" href="#conditional-expressions">6.12. Conditional expressions</a></li>
<li><a class="reference internal" href="#lambda">6.13. Lambdas</a></li>
<li><a class="reference internal" href="#expression-lists">6.14. Expression lists</a></li>
<li><a class="reference internal" href="#evaluation-order">6.15. Evaluation order</a></li>
<li><a class="reference internal" href="#operator-precedence">6.16. Operator precedence</a></li>
</ul>
</li>
</ul>

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