Doc/reference/expressions.rst
.. _expressions:
Expressions
.. index:: expression, BNF
This chapter explains the meaning of the elements of expressions in Python.
Syntax Notes: In this and the following chapters,
:ref:grammar notation <notation> will be used to describe syntax,
not lexical analysis.
When (one alternative of) a syntax rule has the form:
.. productionlist:: python-grammar name: othername
and no semantics are given, the semantics of this form of name are the same
as for othername.
.. _conversions:
.. index:: pair: arithmetic; conversion
When a description of an arithmetic operator below uses the phrase "the numeric
arguments are converted to a common real type", this means that the operator
implementation for built-in numeric types works as described in the
:ref:Numeric Types <stdtypes-mixed-arithmetic> section of the standard
library documentation.
Some additional rules apply for certain operators and non-numeric operands
(for example, a string as a left argument to the % operator).
Extensions must define their own conversion behavior.
.. _atoms:
.. index:: atom
Atoms are the most basic elements of expressions.
The simplest atoms are :ref:names <identifiers> or literals.
Forms enclosed in parentheses, brackets or braces are also categorized
syntactically as atoms.
Formally, the syntax for atoms is:
.. grammar-snippet:: :group: python-grammar
atom:
| 'True'
| 'False'
| 'None'
| '...'
| identifier
| literal
| enclosure
enclosure:
| parenth_form
| list_display
| dict_display
| set_display
| generator_expression
| yield_atom
.. _atom-singletons:
The keywords True, False, and None name
:ref:built-in constants <built-in-consts>.
The token ... names the :py:data:Ellipsis constant.
Evaluation of these atoms yields the corresponding value.
.. note::
Several more built-in constants are available as global variables,
but only the ones mentioned here are :ref:keywords <keywords>.
In particular, these names cannot be reassigned or used as attributes:
.. code-block:: pycon
>>> False = 123
File "<input>", line 1
False = 123
^^^^^
SyntaxError: cannot assign to False
.. _atom-identifiers:
.. index:: name, identifier
An identifier occurring as an atom is a name. See section :ref:identifiers
for lexical definition and section :ref:naming for documentation of naming and
binding.
.. index:: pair: exception; NameError
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 :exc:NameError
exception.
.. _private-name-mangling:
.. index:: pair: name; mangling pair: private; names
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 :dfn:private name of that class.
.. seealso::
The :ref:class specifications <class>.
More precisely, private names are transformed to a longer form before code is generated for them. If the transformed name is longer than 255 characters, implementation-defined truncation may happen.
The transformation is independent of the syntactical context in which the identifier is used but only the following private identifiers are mangled:
Any name used as the name of a variable that is assigned or read or any name of an attribute being accessed.
The :attr:~definition.__name__ attribute of nested functions, classes, and
type aliases is however not mangled.
The name of imported modules, e.g., __spam in import __spam.
If the module is part of a package (i.e., its name contains a dot),
the name is not mangled, e.g., the __foo in import __foo.bar
is not mangled.
The name of an imported member, e.g., __f in from spam import __f.
The transformation rule is defined as follows:
The class name, with leading underscores removed and a single leading
underscore inserted, is inserted in front of the identifier, e.g., the
identifier __spam occurring in a class named Foo, _Foo or
__Foo is transformed to _Foo__spam.
If the class name consists only of underscores, the transformation is the
identity, e.g., the identifier __spam occurring in a class named _
or __ is left as is.
.. _atom-literals:
.. index:: single: literal
A :dfn:literal is a textual representation of a value.
Python supports numeric, string and bytes literals.
:ref:Format strings <f-strings> and :ref:template strings <t-strings>
are treated as string literals.
Numeric literals consist of a single :token:NUMBER <python-grammar:NUMBER>
token, which names an integer, floating-point number, or an imaginary number.
See the :ref:numbers section in Lexical analysis documentation for details.
String and bytes literals may consist of several tokens.
See section :ref:string-concatenation for details.
Note that negative and complex numbers, like -3 or 3+4.2j,
are syntactically not literals, but :ref:unary <unary> or
:ref:binary <binary> arithmetic operations involving the - or +
operator.
Evaluation of a literal yields an object of the given type
(:class:int, :class:float, :class:complex, :class:str,
:class:bytes, or :class:~string.templatelib.Template) with the given value.
The value may be approximated in the case of floating-point
and imaginary literals.
The formal grammar for literals is:
.. grammar-snippet:: :group: python-grammar
literal: strings | NUMBER
.. index:: triple: immutable; data; type pair: immutable; object
Literals and object identity ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.
.. admonition:: CPython implementation detail
For example, in CPython, small integers with the same value evaluate to the same object::
>>> x = 7
>>> y = 7
>>> x is y
True
However, large integers evaluate to different objects::
>>> x = 123456789
>>> y = 123456789
>>> x is y
False
This behavior may change in future versions of CPython. In particular, the boundary between "small" and "large" integers has already changed in the past.
CPython will emit a :py:exc:SyntaxWarning when you compare literals
using is::
>>> x = 7
>>> x is 7
<input>:1: SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True
See :ref:faq-identity-with-is for more information.
:ref:Template strings <t-strings> are immutable but may reference mutable
objects as :class:~string.templatelib.Interpolation values.
For the purposes of this section, two t-strings have the "same value" if
both their structure and the identity of the values match.
.. impl-detail::
Currently, each evaluation of a template string results in a different object.
.. _string-concatenation:
String literal concatenation ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Multiple adjacent string or bytes literals, possibly using different quoting conventions, are allowed, and their meaning is the same as their concatenation::
"hello" 'world' "helloworld"
This feature is defined at the syntactical level, so it only works with literals. To concatenate string expressions at run time, the '+' operator may be used::
greeting = "Hello" space = " " name = "Blaise" print(greeting + space + name) # not: print(greeting space name) Hello Blaise
Literal concatenation can freely mix raw strings, triple-quoted strings, and formatted string literals. For example::
"Hello" r', ' f"{name}!" "Hello, Blaise!"
This feature can be used to reduce the number of backslashes needed, to split long strings conveniently across long lines, or even to add comments to parts of strings. For example::
re.compile("[A-Za-z_]" # letter or underscore "[A-Za-z0-9_]*" # letter, digit or underscore )
However, bytes literals may only be combined with other byte literals; not with string literals of any kind. Also, template string literals may only be combined with other template string literals::
t"Hello" t"{name}!" Template(strings=('Hello', '!'), interpolations=(...))
Formally:
.. grammar-snippet:: :group: python-grammar
strings: (STRING | fstring)+ | tstring+
.. _parenthesized:
.. index:: single: parenthesized form single: () (parentheses); tuple display
A parenthesized form is an optional expression list enclosed in parentheses:
.. productionlist:: python-grammar
parenth_form: "(" [starred_expression] ")"
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.
.. index:: pair: empty; tuple
An empty pair of parentheses yields an empty tuple object. Since tuples are immutable, the same rules as for literals apply (i.e., two occurrences of the empty tuple may or may not yield the same object).
.. index:: single: comma single: , (comma)
Note that tuples are not formed by the parentheses, but rather by use of the comma. 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.
.. _comprehensions:
.. index:: single: comprehensions
For constructing a list, a set or a dictionary Python provides special syntax called "displays", each of them in two flavors:
either the container contents are listed explicitly, or
they are computed via a set of looping and filtering instructions, called a
:dfn:comprehension.
.. index:: single: for; in comprehensions single: if; in comprehensions single: async for; in comprehensions
Common syntax elements for comprehensions are:
.. productionlist:: python-grammar
comprehension: flexible_expression comp_for
comp_for: ["async"] "for" target_list "in" or_test [comp_iter]
comp_iter: comp_for | comp_if
comp_if: "if" or_test [comp_iter]
The comprehension consists of a single expression followed by at least one
:keyword:!for clause and zero or more :keyword:!for or :keyword:!if
clauses. In this case, the elements of the new container are those that would
be produced by considering each of the :keyword:!for or :keyword:!if
clauses a block, nesting from left to right, and evaluating the expression to
produce an element each time the innermost block is reached. If the expression
is starred, the result will instead be unpacked to produce zero or more
elements.
However, aside from the iterable expression in the leftmost :keyword:!for clause,
the comprehension is executed in a separate implicitly nested scope. This ensures
that names assigned to in the target list don't "leak" into the enclosing scope.
The iterable expression in the leftmost :keyword:!for clause is evaluated
directly in the enclosing scope and then passed as an argument to the implicitly
nested scope. Subsequent :keyword:!for clauses and any filter condition in the
leftmost :keyword:!for clause cannot be evaluated in the enclosing scope as
they may depend on the values obtained from the leftmost iterable. For example:
[x*y for x in range(10) for y in range(x, x+10)].
To ensure the comprehension always results in a container of the appropriate
type, yield and yield from expressions are prohibited in the implicitly
nested scope.
.. index:: single: await; in comprehensions
Since Python 3.6, in an :keyword:async def function, an :keyword:!async for
clause may be used to iterate over a :term:asynchronous iterator.
A comprehension in an :keyword:!async def function may consist of either a
:keyword:!for or :keyword:!async for clause following the leading
expression, may contain additional :keyword:!for or :keyword:!async for
clauses, and may also use :keyword:await expressions.
If a comprehension contains :keyword:!async for clauses, or if it contains
:keyword:!await expressions or other asynchronous comprehensions anywhere except
the iterable expression in the leftmost :keyword:!for clause, it is called an
:dfn:asynchronous comprehension. An asynchronous comprehension may suspend the
execution of the coroutine function in which it appears.
See also :pep:530.
.. versionadded:: 3.6 Asynchronous comprehensions were introduced.
.. versionchanged:: 3.8
yield and yield from prohibited in the implicitly nested scope.
.. versionchanged:: 3.11 Asynchronous comprehensions are now allowed inside comprehensions in asynchronous functions. Outer comprehensions implicitly become asynchronous.
.. versionchanged:: 3.15
Unpacking with the * operator is now allowed in the expression.
.. _lists:
.. index:: pair: list; display pair: list; comprehensions pair: empty; list pair: object; list single: [] (square brackets); list expression single: , (comma); expression list
A list display is a possibly empty series of expressions enclosed in square brackets:
.. productionlist:: python-grammar
list_display: "[" [flexible_expression_list | comprehension] "]"
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.
.. _set:
.. index:: pair: set; display pair: set; comprehensions pair: object; set single: {} (curly brackets); set expression single: , (comma); expression list
A set display is denoted by curly braces and distinguishable from dictionary displays by the lack of colons separating keys and values:
.. productionlist:: python-grammar
set_display: "{" (flexible_expression_list | comprehension) "}"
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 {}; this literal constructs an empty
dictionary.
.. _dict:
.. index:: pair: dictionary; display pair: dictionary; comprehensions key, value, key/value pair pair: object; dictionary single: {} (curly brackets); dictionary expression single: : (colon); in dictionary expressions single: , (comma); in dictionary displays
A dictionary display is a possibly empty series of dict items (key/value pairs) enclosed in curly braces:
.. productionlist:: python-grammar
dict_display: "{" [dict_item_list | dict_comprehension] "}"
dict_item_list: dict_item ("," dict_item)* [","]
dict_comprehension: dict_item comp_for
dict_item: expression ":" expression | "**" or_expr
A dictionary display yields a new dictionary object.
If a comma-separated sequence of dict items 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 value. This means that you can specify the same key multiple times in the dict item list, and the final dictionary's value for that key will be the last one given.
.. index:: unpacking; dictionary single: **; in dictionary displays
A double asterisk ** denotes :dfn:dictionary unpacking.
Its operand must be a :term:mapping. Each mapping item is added
to the new dictionary. Later values replace values already set by
earlier dict items and earlier dictionary unpackings.
.. versionadded:: 3.5
Unpacking into dictionary displays, originally proposed by :pep:448.
A dict comprehension may take one of two forms:
The first form uses 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.
The second form uses a single expression prefixed by the ** dictionary
unpacking operator followed by the usual "for" and "if" clauses. When the
comprehension is evaluated, the expression is evaluated and then unpacked,
inserting zero or more key/value pairs into the new dictionary.
Both forms of dictionary comprehension retain the property that if the same key is specified multiple times, the associated value in the resulting dictionary will be the last one specified.
.. index:: pair: immutable; object hashable
Restrictions on the types of the key values are listed earlier in section
:ref:types. (To summarize, the key type should be :term:hashable, which excludes
all mutable objects.) Clashes between duplicate keys are not detected; the last
value (textually rightmost in the display) stored for a given key value
prevails.
.. versionchanged:: 3.8
Prior to Python 3.8, in dict comprehensions, the evaluation order of key
and value was not well-defined. In CPython, the value was evaluated before
the key. Starting with 3.8, the key is evaluated before the value, as
proposed by :pep:572.
.. versionchanged:: 3.15
Unpacking with the ** operator is now allowed in dictionary comprehensions.
.. _genexpr:
.. index:: pair: generator; expression pair: object; generator single: () (parentheses); generator expression
A generator expression is a compact generator notation in parentheses:
.. productionlist:: python-grammar
generator_expression: "(" flexible_expression comp_for ")"
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
:meth:~generator.__next__ method is called for the generator object (in the same
fashion as normal generators). However, the iterable expression in the
leftmost :keyword:!for clause is immediately evaluated, and the
:term:iterator is immediately created for that iterable, so that an error
produced while creating the iterator will be emitted at the point where the generator expression
is defined, rather than at the point where the first value is retrieved.
Subsequent :keyword:!for clauses and any filter condition in the leftmost
:keyword:!for clause cannot be evaluated in the enclosing scope as they may
depend on the values obtained from the leftmost iterable. For example:
(x*y for x in range(10) for y in range(x, x+10)).
The parentheses can be omitted on calls with only one argument. See section
:ref:calls for details.
To avoid interfering with the expected operation of the generator expression
itself, yield and yield from expressions are prohibited in the
implicitly defined generator.
If a generator expression contains either :keyword:!async for
clauses or :keyword:await expressions it is called an
:dfn:asynchronous generator expression. An asynchronous generator
expression returns a new asynchronous generator object,
which is an asynchronous iterator (see :ref:async-iterators).
.. versionadded:: 3.6 Asynchronous generator expressions were introduced.
.. versionchanged:: 3.7
Prior to Python 3.7, asynchronous generator expressions could
only appear in :keyword:async def coroutines. Starting
with 3.7, any function can use asynchronous generator expressions.
.. versionchanged:: 3.8
yield and yield from prohibited in the implicitly nested scope.
.. _yieldexpr:
.. index:: pair: keyword; yield pair: keyword; from pair: yield; expression pair: generator; function
.. productionlist:: python-grammar
yield_atom: "(" yield_expression ")"
yield_from: "yield" "from" expression
yield_expression: "yield" yield_list | yield_from
The yield expression is used when defining a :term:generator function
or an :term:asynchronous generator 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 function,
and using it in an :keyword:async def function's body causes that
coroutine function to be an asynchronous generator function. For example::
def gen(): # defines a generator function
yield 123
async def agen(): # defines an asynchronous generator function
yield 123
Due to their side effects on the containing scope, yield expressions
are not permitted as part of the implicitly defined scopes used to
implement comprehensions and generator expressions.
.. versionchanged:: 3.8 Yield expressions prohibited in the implicitly nested scopes used to implement comprehensions and generator expressions.
Generator functions are described below, while asynchronous generator
functions are described separately in section
:ref:asynchronous-generator-functions.
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 :token:~python-grammar:yield_list
to the generator's caller,
or None if :token:~python-grammar:yield_list is omitted.
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 :meth:~generator.__next__ is used
(typically via either a :keyword:for or the :func:next builtin) then the
result is :const:None. Otherwise, if :meth:~generator.send is used, then
the result will be the value passed in to that method.
.. index:: single: coroutine
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 :keyword:try 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 :meth:~generator.close method will be called,
allowing any pending :keyword:finally clauses to execute.
.. index:: single: from; yield from expression
When yield from <expr> is used, the supplied expression must be an
iterable. The values produced by iterating that iterable are passed directly
to the caller of the current generator's methods. Any values passed in with
:meth:~generator.send and any exceptions passed in with
:meth:~generator.throw are passed to the underlying iterator if it has the
appropriate methods. If this is not the case, then :meth:~generator.send
will raise :exc:AttributeError or :exc:TypeError, while
:meth:~generator.throw will just raise the passed in exception immediately.
When the underlying iterator is complete, the :attr:~StopIteration.value
attribute of the raised :exc:StopIteration instance becomes the value of
the yield expression. It can be either set explicitly when raising
:exc:StopIteration, or automatically when the subiterator is a generator
(by returning a value from the subgenerator).
.. versionchanged:: 3.3
Added yield from <expr> to delegate control flow to a subiterator.
The parentheses may be omitted when the yield expression is the sole expression on the right hand side of an assignment statement.
.. seealso::
:pep:255 - Simple Generators
The proposal for adding generators and the :keyword:yield statement to Python.
:pep:342 - Coroutines via Enhanced Generators
The proposal to enhance the API and syntax of generators, making them
usable as simple coroutines.
:pep:380 - Syntax for Delegating to a Subgenerator
The proposal to introduce the :token:~python-grammar:yield_from syntax,
making delegation to subgenerators easy.
:pep:525 - Asynchronous Generators
The proposal that expanded on :pep:492 by adding generator capabilities to
coroutine functions.
.. index:: pair: object; generator .. _generator-methods:
Generator-iterator methods ^^^^^^^^^^^^^^^^^^^^^^^^^^
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 :exc:ValueError exception.
.. index:: pair: exception; StopIteration
.. method:: generator.next()
Starts the execution of a generator function or resumes it at the last
executed yield expression. When a generator function is resumed with a
:meth:~generator.__next__ method, the current yield expression always
evaluates to :const:None. The execution then continues to the next yield
expression, where the generator is suspended again, and the value of the
:token:~python-grammar:yield_list is returned to :meth:__next__'s
caller. If the generator exits without yielding another value, a
:exc:StopIteration exception is raised.
This method is normally called implicitly, e.g. by a :keyword:for loop, or
by the built-in :func:next function.
.. method:: generator.send(value)
Resumes the execution and "sends" a value into the generator function. The
value argument becomes the result of the current yield expression. The
:meth:send method returns the next value yielded by the generator, or
raises :exc:StopIteration if the generator exits without yielding another
value. When :meth:send is called to start the generator, it must be called
with :const:None as the argument, because there is no yield expression that
could receive the value.
.. method:: generator.throw(value) generator.throw(type[, value[, traceback]])
Raises an exception 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 :exc:StopIteration 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.
In typical use, this is called with a single exception instance similar to the
way the :keyword:raise keyword is used.
For backwards compatibility, however, the second signature is
supported, following a convention from older versions of Python.
The type argument should be an exception class, and value
should be an exception instance. If the value is not provided, the
type constructor is called to get an instance. If traceback
is provided, it is set on the exception, otherwise any existing
:attr:~BaseException.__traceback__ attribute stored in value may
be cleared.
.. versionchanged:: 3.12
The second signature \(type\[, value\[, traceback\]\]\) is deprecated and
may be removed in a future version of Python.
.. index:: pair: exception; GeneratorExit
.. method:: generator.close()
Raises a :exc:GeneratorExit exception at the point where the generator
function was paused (equivalent to calling throw(GeneratorExit)).
The exception is raised by the yield expression where the generator was paused.
If the generator function catches the exception and returns a
value, this value is returned from :meth:close. If the generator function
is already closed, or raises :exc:GeneratorExit (by not catching the
exception), :meth:close returns :const:None. If the generator yields a
value, a :exc:RuntimeError is raised. If the generator raises any other
exception, it is propagated to the caller. If the generator has already
exited due to an exception or normal exit, :meth:close returns
:const:None and has no other effect.
.. versionchanged:: 3.13
If a generator returns a value upon being closed, the value is returned
by :meth:`close`.
.. index:: single: yield; examples
Examples ^^^^^^^^
Here is a simple example that demonstrates the behavior of generators and generator functions::
def echo(value=None): ... print("Execution starts when 'next()' is called for the first time.") ... try: ... while True: ... try: ... value = (yield value) ... except Exception as e: ... value = e ... finally: ... print("Don't forget to clean up when 'close()' is called.") ... generator = echo(1) print(next(generator)) Execution starts when 'next()' is called for the first time. 1 print(next(generator)) None print(generator.send(2)) 2 generator.throw(TypeError, "spam") TypeError('spam',) generator.close() Don't forget to clean up when 'close()' is called.
For examples using yield from, see :ref:pep-380 in "What's New in
Python."
.. _asynchronous-generator-functions:
Asynchronous generator functions ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The presence of a yield expression in a function or method defined using
:keyword:async def further defines the function as an
:term:asynchronous generator 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
:keyword:async for statement in a coroutine function analogously to
how a generator object would be used in a :keyword:for statement.
Calling one of the asynchronous generator's methods returns an :term:awaitable
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 :token:~python-grammar:yield_list 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
:meth:~agen.__anext__ is used then the result is :const:None. Otherwise, if
:meth:~agen.asend is used, then the result will be the value passed in to that
method.
If an asynchronous generator happens to exit early by :keyword:break, the caller
task being cancelled, or other exceptions, the generator's async cleanup code
will run and possibly raise exceptions or access context variables in an
unexpected context--perhaps after the lifetime of tasks it depends, or
during the event loop shutdown when the async-generator garbage collection hook
is called.
To prevent this, the caller must explicitly close the async generator by calling
:meth:~agen.aclose method to finalize the generator and ultimately detach it
from the event loop.
In an asynchronous generator function, yield expressions are allowed anywhere
in a :keyword:try 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 :keyword:!try
construct could result in a failure to execute pending :keyword:finally
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 :meth:~agen.aclose method and run the resulting
coroutine object, thus allowing any pending :keyword:!finally clauses
to execute.
To take care of finalization upon event loop termination, an event loop should
define a finalizer function which takes an asynchronous generator-iterator and
presumably calls :meth:~agen.aclose and executes the coroutine.
This finalizer may be registered by calling :func:sys.set_asyncgen_hooks.
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
asyncio.Loop.shutdown_asyncgens in :source:Lib/asyncio/base_events.py.
The expression yield from <expr> is a syntax error when used in an
asynchronous generator function.
.. index:: pair: object; asynchronous-generator .. _asynchronous-generator-methods:
Asynchronous generator-iterator methods ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This subsection describes the methods of an asynchronous generator iterator, which are used to control the execution of a generator function.
.. index:: pair: exception; StopAsyncIteration
.. method:: agen.anext() :async:
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 an :meth:~agen.__anext__
method, the current yield expression always evaluates to :const:None in the
returned awaitable, which when run will continue to the next yield
expression. The value of the :token:~python-grammar:yield_list of the
yield expression is the value of the :exc:StopIteration exception raised by
the completing coroutine. If the asynchronous generator exits without
yielding another value, the awaitable instead raises a
:exc:StopAsyncIteration exception, signalling that the asynchronous
iteration has completed.
This method is normally called implicitly by a :keyword:async for loop.
.. method:: agen.asend(value) :async:
Returns an awaitable which when run resumes the execution of the
asynchronous generator. As with the :meth:~generator.send 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 :meth:asend method will return the next
value yielded by the generator as the value of the raised
:exc:StopIteration, or raises :exc:StopAsyncIteration if the
asynchronous generator exits without yielding another value. When
:meth:asend is called to start the asynchronous
generator, it must be called with :const:None as the argument,
because there is no yield expression that could receive the value.
.. method:: agen.athrow(value) agen.athrow(type[, value[, traceback]]) :async:
Returns an awaitable that raises an exception of type type 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
:exc:StopIteration exception. If the asynchronous generator exits
without yielding another value, a :exc:StopAsyncIteration 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.
.. versionchanged:: 3.12
The second signature \(type\[, value\[, traceback\]\]\) is deprecated and
may be removed in a future version of Python.
.. index:: pair: exception; GeneratorExit
.. method:: agen.aclose() :async:
Returns an awaitable that when run will throw a :exc:GeneratorExit 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 :exc:GeneratorExit (by not catching the exception),
then the returned awaitable will raise a :exc:StopIteration exception.
Any further awaitables returned by subsequent calls to the asynchronous
generator will raise a :exc:StopAsyncIteration exception. If the
asynchronous generator yields a value, a :exc:RuntimeError 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 :meth:aclose will return an awaitable that does nothing.
.. _primaries:
.. index:: single: primary
Primaries represent the most tightly bound operations of the language. Their syntax is:
.. productionlist:: python-grammar
primary: atom | attributeref | subscription | call
.. _attribute-references:
.. index:: pair: attribute; reference single: . (dot); attribute reference
An attribute reference is a primary followed by a period and a name:
.. productionlist:: python-grammar
attributeref: primary "." identifier
.. index:: pair: exception; AttributeError pair: object; module pair: object; list
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. The type and value produced is determined by the object. Multiple evaluations of the same attribute reference may yield different objects.
This production can be customized by overriding the
:meth:~object.__getattribute__ method or the :meth:~object.__getattr__
method. The :meth:!__getattribute__ method is called first and either
returns a value or raises :exc:AttributeError if the attribute is not
available.
If an :exc:AttributeError is raised and the object has a :meth:!__getattr__
method, that method is called as a fallback.
.. _subscriptions:
.. index:: single: subscription single: [] (square brackets); subscription
.. index:: pair: object; sequence pair: object; mapping pair: object; string pair: object; tuple pair: object; list pair: object; dictionary pair: sequence; item
The :dfn:subscription syntax is usually used for selecting an element from a
:ref:container <sequence-types> -- for example, to get a value from
a :class:dict::
digits_by_name = {'one': 1, 'two': 2} digits_by_name['two'] # Subscripting a dictionary using the key 'two' 2
In the subscription syntax, the object being subscribed -- a
:ref:primary <primaries> -- is followed by a :dfn:subscript in
square brackets.
In the simplest case, the subscript is a single expression.
Depending on the type of the object being subscribed, the subscript is
sometimes called a :term:key (for mappings), :term:index (for sequences),
or type argument (for :term:generic types <generic type>).
Syntactically, these are all equivalent::
colors = ['red', 'blue', 'green', 'black'] colors[3] # Subscripting a list using the index 3 'black'
list[str] # Parameterizing the list type using the type argument str list[str]
At runtime, the interpreter will evaluate the primary and
the subscript, and call the primary's :meth:~object.__getitem__ or
:meth:~object.__class_getitem__ :term:special method with the subscript
as argument.
For more details on which of these methods is called, see
:ref:classgetitem-versus-getitem.
To show how subscription works, we can define a custom object that
implements :meth:~object.__getitem__ and prints out the value of
the subscript::
class SubscriptionDemo: ... def getitem(self, key): ... print(f'subscripted with: {key!r}') ... demo = SubscriptionDemo() demo[1] subscripted with: 1 demo['a' * 3] subscripted with: 'aaa'
See :meth:~object.__getitem__ documentation for how built-in types handle
subscription.
Subscriptions may also be used as targets in :ref:assignment <assignment> or
:ref:deletion <del> statements.
In these cases, the interpreter will call the subscripted object's
:meth:~object.__setitem__ or :meth:~object.__delitem__
:term:special method, respectively, instead of :meth:~object.__getitem__.
.. code-block::
colors = ['red', 'blue', 'green', 'black'] colors[3] = 'white' # Setting item at index colors ['red', 'blue', 'green', 'white'] del colors[3] # Deleting item at index 3 colors ['red', 'blue', 'green']
All advanced forms of subscript documented in the following sections are also usable for assignment and deletion.
.. index:: single: slicing single: slice single: : (colon); slicing single: , (comma); slicing
.. index:: pair: object; sequence pair: object; string pair: object; tuple pair: object; list
.. _slicings:
Slicings ^^^^^^^^
A more advanced form of subscription, :dfn:slicing, is commonly used
to extract a portion of a :ref:sequence <datamodel-sequences>.
In this form, the subscript is a :term:slice: up to three
expressions separated by colons.
Any of the expressions may be omitted, but a slice must contain at least one
colon::
number_names = ['zero', 'one', 'two', 'three', 'four', 'five'] number_names[1:3] ['one', 'two'] number_names[1:] ['one', 'two', 'three', 'four', 'five'] number_names[:3] ['zero', 'one', 'two'] number_names[:] ['zero', 'one', 'two', 'three', 'four', 'five'] number_names[::2] ['zero', 'two', 'four'] number_names[:-3] ['zero', 'one', 'two'] del number_names[4:] number_names ['zero', 'one', 'two', 'three']
When a slice is evaluated, the interpreter constructs a :class:slice object
whose :attr:~slice.start, :attr:~slice.stop and
:attr:~slice.step attributes, respectively, are the results of the
expressions between the colons.
Any missing expression evaluates to :const:None.
This :class:!slice object is then passed to the :meth:~object.__getitem__
or :meth:~object.__class_getitem__ :term:special method, as above. ::
demo[2:3] subscripted with: slice(2, 3, None) demo[::'spam'] subscripted with: slice(None, None, 'spam')
Comma-separated subscripts ^^^^^^^^^^^^^^^^^^^^^^^^^^
The subscript can also be given as two or more comma-separated expressions or slices::
demo[1, 2, 3] subscripted with: (1, 2, 3) demo[1:2, 3] subscripted with: (slice(1, 2, None), 3)
This form is commonly used with numerical libraries for slicing
multi-dimensional data.
In this case, the interpreter constructs a :class:tuple of the results of the
expressions or slices, and passes this tuple to the :meth:~object.__getitem__
or :meth:~object.__class_getitem__ :term:special method, as above.
The subscript may also be given as a single expression or slice followed by a comma, to specify a one-element tuple::
demo['spam',] subscripted with: ('spam',)
"Starred" subscriptions ^^^^^^^^^^^^^^^^^^^^^^^
.. versionadded:: 3.11
Expressions in tuple_slices may be starred. See :pep:646.
The subscript can also contain a starred expression.
In this case, the interpreter unpacks the result into a tuple, and passes
this tuple to :meth:~object.__getitem__ or :meth:~object.__class_getitem__::
demo[*range(10)] subscripted with: (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
Starred expressions may be combined with comma-separated expressions and slices::
demo['a', 'b', *range(3), 'c'] subscripted with: ('a', 'b', 0, 1, 2, 'c')
Formal subscription grammar ^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. grammar-snippet:: :group: python-grammar
subscription: primary '[' subscript ']'
subscript: single_subscript | tuple_subscript
single_subscript: proper_slice | assignment_expression
proper_slice: [expression] ":" [expression] [ ":" [expression] ]
tuple_subscript: ','.(single_subscript | starred_expression)+ [',']
Recall that the | operator :ref:denotes ordered choice <notation>.
Specifically, in :token:!subscript, if both alternatives would match, the
first (:token:!single_subscript) has priority.
.. index:: pair: object; callable single: call single: argument; call semantics single: () (parentheses); call single: , (comma); argument list single: = (equals); in function calls
.. _calls:
A call calls a callable object (e.g., a :term:function) with a possibly empty
series of :term:arguments <argument>:
.. productionlist:: python-grammar
call: primary "(" [argument_list [","] | comprehension] ")"
argument_list: positional_arguments ["," starred_and_keywords]
: ["," keywords_arguments]
: | starred_and_keywords ["," keywords_arguments]
: | keywords_arguments
positional_arguments: positional_item ("," positional_item)*
positional_item: assignment_expression | "" expression
starred_and_keywords: ("" expression | keyword_item)
: ("," "" expression | "," keyword_item)
keywords_arguments: (keyword_item | "" expression)
: ("," keyword_item | "," "" expression)*
keyword_item: identifier "=" expression
An optional trailing comma may be present after the positional and keyword arguments but does not affect the semantics.
.. index:: single: parameter; call 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 :meth:~object.__call__ method are callable). All
argument expressions are evaluated before the call is attempted. Please refer
to section :ref:function for the syntax of formal :term:parameter lists.
.. XXX update with kwonly args PEP
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 :exc:TypeError exception is raised. Otherwise, the
argument is placed in the slot, filling it (even if the expression is
None, 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 :exc:TypeError exception is
raised. Otherwise, the list of filled slots is used as the argument list for
the call.
.. impl-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 :c:func:PyArg_ParseTuple to
parse their arguments.
If there are more positional arguments than there are formal parameter slots, a
:exc:TypeError exception is raised, unless a formal parameter using the syntax
*identifier 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
:exc:TypeError exception is raised, unless a formal parameter using the syntax
**identifier 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.
.. index:: single: * (asterisk); in function calls single: unpacking; in function calls
If the syntax *expression appears in the function call, expression must
evaluate to an :term:iterable. Elements from these iterables are
treated as if they were additional positional arguments. For the call
f(x1, x2, *y, x3, x4), 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 *expression syntax may appear
after explicit keyword arguments, it is processed before the
keyword arguments (and any **expression arguments -- see below). So::
def f(a, b): ... print(a, b) ... f(b=1, *(2,)) 2 1 f(a=1, *(2,)) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: f() got multiple values for keyword argument 'a' f(1, *(2,)) 1 2
It is unusual for both keyword arguments and the *expression syntax to be
used in the same call, so in practice this confusion does not often arise.
.. index:: single: **; in function calls
If the syntax **expression appears in the function call, expression must
evaluate to a :term:mapping, the contents of which are treated as
additional keyword arguments. If a parameter matching a key has already been
given a value (by an explicit keyword argument, or from another unpacking),
a :exc:TypeError exception is raised.
When **expression is used, each key in this mapping must be
a string.
Each value from the mapping is assigned to the first formal parameter
eligible for keyword assignment whose name is equal to the key.
A key need not be a Python identifier (e.g. "max-temp °F" is acceptable,
although it will not match any formal parameter that could be declared).
If there is no match to a formal parameter
the key-value pair is collected by the ** parameter, if there is one,
or if there is not, a :exc:TypeError exception is raised.
Formal parameters using the syntax *identifier or **identifier cannot be
used as positional argument slots or as keyword argument names.
.. versionchanged:: 3.5
Function calls accept any number of * and ** unpackings,
positional arguments may follow iterable unpackings (*),
and keyword arguments may follow dictionary unpackings (**).
Originally proposed by :pep:448.
A call always returns some value, possibly None, unless it raises an
exception. How this value is computed depends on the type of the callable
object.
If it is---
a user-defined function: .. index:: pair: function; call triple: user-defined; function; call pair: object; user-defined function pair: object; function
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 :ref:function. When the code block
executes a :keyword:return statement, this specifies the return value of the
function call. If execution reaches the end of the code block without
executing a :keyword:return statement, the return value is None.
a built-in function or method: .. index:: pair: function; call pair: built-in function; call pair: method; call pair: built-in method; call pair: object; built-in method pair: object; built-in function pair: object; method pair: object; function
The result is up to the interpreter; see :ref:built-in-funcs for the
descriptions of built-in functions and methods.
a class object: .. index:: pair: object; class pair: class object; call
A new instance of that class is returned.
a class instance method: .. index:: pair: object; class instance pair: object; instance pair: class instance; call
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.
a class instance: .. index:: pair: instance; call single: call() (object method)
The class must define a :meth:~object.__call__ method; the effect is then the same as
if that method was called.
.. index:: pair: keyword; await .. _await:
Suspend the execution of :term:coroutine on an :term:awaitable object.
Can only be used inside a :term:coroutine function.
.. productionlist:: python-grammar
await_expr: "await" primary
.. versionadded:: 3.5
.. _power:
.. index:: pair: power; operation pair: operator; **
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:
.. productionlist:: python-grammar
power: (await_expr | primary) ["**" u_expr]
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): -1**2 results in -1.
The power operator has the same semantics as the built-in :func:pow function,
when called with two arguments: it yields its left argument raised to the power
of its right argument.
Numeric arguments are first :ref:converted to a common type <stdtypes-mixed-arithmetic>,
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, 10**2 returns 100, but
10**-2 returns 0.01.
Raising 0.0 to a negative power results in a :exc:ZeroDivisionError.
Raising a negative number to a fractional power results in a :class:complex
number. (In earlier versions it raised a :exc:ValueError.)
This operation can be customized using the special :meth:~object.__pow__ and
:meth:~object.__rpow__ methods.
.. _unary:
.. index:: triple: unary; arithmetic; operation triple: unary; bitwise; operation
All unary arithmetic and bitwise operations have the same priority:
.. productionlist:: python-grammar
u_expr: power | "-" u_expr | "+" u_expr | "~" u_expr
.. index:: single: negation single: minus single: operator; - (minus) single: - (minus); unary operator
The unary - (minus) operator yields the negation of its numeric argument; the
operation can be overridden with the :meth:~object.__neg__ special method.
.. index:: single: plus single: operator; + (plus) single: + (plus); unary operator
The unary + (plus) operator yields its numeric argument unchanged; the
operation can be overridden with the :meth:~object.__pos__ special method.
.. index:: single: inversion pair: operator; ~ (tilde)
The unary ~ (invert) operator yields the bitwise inversion of its integer
argument. The bitwise inversion of x is defined as -(x+1). It only
applies to integral numbers or to custom objects that override the
:meth:~object.__invert__ special method.
.. index:: pair: exception; TypeError
In all three cases, if the argument does not have the proper type, a
:exc:TypeError exception is raised.
.. _binary:
.. index:: triple: binary; arithmetic; operation
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:
.. productionlist:: python-grammar
m_expr: u_expr | m_expr "*" u_expr | m_expr "@" m_expr |
: m_expr "//" u_expr | m_expr "/" u_expr |
: m_expr "%" u_expr
a_expr: m_expr | a_expr "+" m_expr | a_expr "-" m_expr
.. index:: single: multiplication pair: operator; * (asterisk)
The * (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
:ref:converted to a common real type <stdtypes-mixed-arithmetic> and then
multiplied together. In the latter case, sequence repetition is performed;
a negative repetition factor yields an empty sequence.
This operation can be customized using the special :meth:~object.__mul__ and
:meth:~object.__rmul__ methods.
.. versionchanged:: 3.14 If only one operand is a complex number, the other operand is converted to a floating-point number.
.. index:: single: matrix multiplication pair: operator; @ (at)
The @ (at) operator is intended to be used for matrix multiplication. No
builtin Python types implement this operator.
This operation can be customized using the special :meth:~object.__matmul__ and
:meth:~object.__rmatmul__ methods.
.. versionadded:: 3.5
.. index:: pair: exception; ZeroDivisionError single: division pair: operator; / (slash) pair: operator; //
The / (division) and // (floor division) operators yield the quotient of
their arguments. The numeric arguments are first
:ref:converted to a common type <stdtypes-mixed-arithmetic>.
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 :exc:ZeroDivisionError
exception.
The division operation can be customized using the special :meth:~object.__truediv__
and :meth:~object.__rtruediv__ methods.
The floor division operation can be customized using the special
:meth:~object.__floordiv__ and :meth:~object.__rfloordiv__ methods.
.. index:: single: modulo pair: operator; % (percent)
The % (modulo) operator yields the remainder from the division of the first
argument by the second. The numeric arguments are first
:ref:converted to a common type <stdtypes-mixed-arithmetic>.
A zero right argument raises the :exc:ZeroDivisionError exception. The
arguments may be floating-point numbers, e.g., 3.14%0.7 equals 0.34
(since 3.14 equals 4*0.7 + 0.34.) 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
[#]_.
The floor division and modulo operators are connected by the following
identity: x == (x//y)*y + (x%y). Floor division and modulo are also
connected with the built-in function :func:divmod: divmod(x, y) == (x//y, x%y). [#]_.
In addition to performing the modulo operation on numbers, the % 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 :ref:old-string-formatting.
The modulo operation can be customized using the special :meth:~object.__mod__
and :meth:~object.__rmod__ methods.
The floor division operator, the modulo operator, and the :func:divmod
function are not defined for complex numbers. Instead, convert to a
floating-point number using the :func:abs function if appropriate.
.. index:: single: addition single: operator; + (plus) single: + (plus); binary operator
The + (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
:ref:converted to a common real type <stdtypes-mixed-arithmetic> and then
added together.
In the latter case, the sequences are concatenated.
This operation can be customized using the special :meth:~object.__add__ and
:meth:~object.__radd__ methods.
.. versionchanged:: 3.14 If only one operand is a complex number, the other operand is converted to a floating-point number.
.. index:: single: subtraction single: operator; - (minus) single: - (minus); binary operator
The - (subtraction) operator yields the difference of its arguments.
The numeric arguments are first
:ref:converted to a common real type <stdtypes-mixed-arithmetic>.
This operation can be customized using the special :meth:~object.__sub__ and
:meth:~object.__rsub__ methods.
.. versionchanged:: 3.14 If only one operand is a complex number, the other operand is converted to a floating-point number.
.. _shifting:
.. index:: pair: shifting; operation pair: operator; << pair: operator; >>
The shifting operations have lower priority than the arithmetic operations:
.. productionlist:: python-grammar
shift_expr: a_expr | shift_expr ("<<" | ">>") a_expr
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.
The left shift operation can be customized using the special :meth:~object.__lshift__
and :meth:~object.__rlshift__ methods.
The right shift operation can be customized using the special :meth:~object.__rshift__
and :meth:~object.__rrshift__ methods.
.. index:: pair: exception; ValueError
A right shift by n bits is defined as floor division by pow(2,n). A left
shift by n bits is defined as multiplication with pow(2,n).
.. _bitwise:
.. index:: triple: binary; bitwise; operation
Each of the three bitwise operations has a different priority level:
.. productionlist:: python-grammar
and_expr: shift_expr | and_expr "&" shift_expr
xor_expr: and_expr | xor_expr "^" and_expr
or_expr: xor_expr | or_expr "|" xor_expr
.. index:: pair: bitwise; and pair: operator; & (ampersand)
The & operator yields the bitwise AND of its arguments, which must be
integers or one of them must be a custom object overriding :meth:~object.__and__ or
:meth:~object.__rand__ special methods.
.. index:: pair: bitwise; xor pair: exclusive; or pair: operator; ^ (caret)
The ^ operator yields the bitwise XOR (exclusive OR) of its arguments, which
must be integers or one of them must be a custom object overriding :meth:~object.__xor__ or
:meth:~object.__rxor__ special methods.
.. index:: pair: bitwise; or pair: inclusive; or pair: operator; | (vertical bar)
The | operator yields the bitwise (inclusive) OR of its arguments, which
must be integers or one of them must be a custom object overriding :meth:~object.__or__ or
:meth:~object.__ror__ special methods.
.. _comparisons:
.. index:: single: comparison pair: C; language pair: operator; < (less) pair: operator; > (greater) pair: operator; <= pair: operator; >= pair: operator; == pair: operator; !=
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 a < b < c have the interpretation that is conventional
in mathematics:
.. productionlist:: python-grammar
comparison: or_expr (comp_operator or_expr)*
comp_operator: "<" | ">" | "==" | ">=" | "<=" | "!="
: | "is" ["not"] | ["not"] "in"
Comparisons yield boolean values: True or False. Custom
:dfn:rich comparison methods may return non-boolean values. In this case
Python will call :func:bool on such value in boolean contexts.
.. index:: pair: chaining; comparisons
Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to
x < y and y <= z, except that y is evaluated only once (but in both
cases z is not evaluated at all when x < y is found to be false).
Formally, if a, b, c, ..., y, z are expressions and op1, op2, ...,
opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent
to a op1 b and b op2 c and ... y opN z, except that each expression is
evaluated at most once.
Note that a op1 b op2 c doesn't imply any kind of comparison between a and
c, so that, e.g., x < y > z is perfectly legal (though perhaps not
pretty).
.. _expressions-value-comparisons:
The operators <, >, ==, >=, <=, and != compare the
values of two objects. The objects do not need to have the same type.
Chapter :ref:objects 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 :class:object, they
inherit the default comparison behavior from :class:object. Types can
customize their comparison behavior by implementing
:dfn:rich comparison methods like :meth:~object.__lt__, described in
:ref:customization.
The default behavior for equality comparison (== and !=) 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. x is y
implies x == y).
A default order comparison (<, >, <=, and >=) is not provided;
an attempt raises :exc:TypeError. 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.
Numbers of built-in numeric types (:ref:typesnumeric) and of the standard
library types :class:fractions.Fraction and :class:decimal.Decimal 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 float('NaN') and decimal.Decimal('NaN') are
special. Any ordered comparison of a number to a not-a-number value is false.
A counter-intuitive implication is that not-a-number values are not equal to
themselves. For example, if x = float('NaN'), 3 < x, x < 3 and
x == x are all false, while x != x is true. This behavior is
compliant with IEEE 754.
None and :data:NotImplemented are singletons. :PEP:8 advises that
comparisons for singletons should always be done with is or is not,
never the equality operators.
Binary sequences (instances of :class:bytes or :class:bytearray) can be
compared within and across their types. They compare lexicographically using
the numeric values of their elements.
Strings (instances of :class:str) compare lexicographically using the
numerical Unicode code points (the result of the built-in function
:func:ord) of their characters. [#]_
Strings and binary sequences cannot be directly compared.
Sequences (instances of :class:tuple, :class:list, or :class:range) 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
:exc:TypeError.
Sequences compare lexicographically using comparison of corresponding elements. The built-in containers typically assume identical objects are equal to themselves. That lets them bypass equality tests for identical objects to improve performance and to maintain their internal invariants.
Lexicographical comparison between built-in collections works as follows:
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, [1,2] == (1,2) is false because the type is not the
same).
Collections that support order comparison are ordered the same as their
first unequal elements (for example, [1,2,x] <= [1,2,y] has the same
value as x <= y). If a corresponding element does not exist, the
shorter collection is ordered first (for example, [1,2] < [1,2,3] is
true).
Mappings (instances of :class:dict) compare equal if and only if they have
equal (key, value) pairs. Equality comparison of the keys and values
enforces reflexivity.
Order comparisons (<, >, <=, and >=) raise :exc:TypeError.
Sets (instances of :class:set or :class:frozenset) 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 {1,2} and {2,3}
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, :func:min, :func:max, and
:func:sorted produce undefined results given a list of sets as inputs).
Comparison of sets enforces reflexivity of its elements.
Most other built-in types have no comparison methods implemented, so they inherit the default comparison behavior.
User-defined classes that customize their comparison behavior should follow some consistency rules, if possible:
Equality comparison should be reflexive. In other words, identical objects should compare equal:
x is y implies x == y
Comparison should be symmetric. In other words, the following expressions should have the same result:
x == y and y == x
x != y and y != x
x < y and y > x
x <= y and y >= x
Comparison should be transitive. The following (non-exhaustive) examples illustrate that:
x > y and y > z implies x > z
x < y and y <= z implies x < z
Inverse comparison should result in the boolean negation. In other words, the following expressions should have the same result:
x == y and not x != y
x < y and not x >= y (for total ordering)
x > y and not x <= y (for total ordering)
The last two expressions apply to totally ordered collections (e.g. to
sequences, but not to sets or mappings). See also the
:func:~functools.total_ordering decorator.
The :func:hash result should be consistent with equality.
Objects that are equal should either have the same hash value,
or be marked as unhashable.
Python does not enforce these consistency rules. In fact, the not-a-number values are an example for not following these rules.
.. _in: .. _not in: .. _membership-test-details:
The operators :keyword:in and :keyword:not in test for membership. x in s evaluates to True if x is a member of s, and False otherwise.
x not in s returns the negation of x in s. All built-in sequences and
set types support this as well as dictionary, for which :keyword:!in tests
whether the dictionary has a given key. For container types such as list, tuple,
set, frozenset, dict, or collections.deque, the expression x in y is equivalent
to any(x is e or x == e for e in y).
For the string and bytes types, x in y is True if and only if x is a
substring of y. An equivalent test is y.find(x) != -1. Empty strings are
always considered to be a substring of any other string, so "" in "abc" will
return True.
For user-defined classes which define the :meth:~object.__contains__ method, x in y returns True if y.__contains__(x) returns a true value, and
False otherwise.
For user-defined classes which do not define :meth:~object.__contains__ but do define
:meth:~object.__iter__, x in y is True if some value z, for which the
expression x is z or x == z is true, is produced while iterating over y.
If an exception is raised during the iteration, it is as if :keyword:in raised
that exception.
Lastly, the old-style iteration protocol is tried: if a class defines
:meth:~object.__getitem__, x in y is True if and only if there is a non-negative
integer index i such that x is y[i] or x == y[i], and no lower integer index
raises the :exc:IndexError exception. (If any other exception is raised, it is as
if :keyword:in raised that exception).
.. index:: pair: operator; in pair: operator; not in pair: membership; test pair: object; sequence
The operator :keyword:not in is defined to have the inverse truth value of
:keyword:in.
.. index:: pair: operator; is pair: operator; is not pair: identity; test
.. _is: .. _is not:
The operators :keyword:is and :keyword:is not test for an object's identity: x is y is true if and only if x and y are the same object. An Object's identity
is determined using the :meth:id function. x is not y yields the inverse
truth value. [#]_
.. _booleans: .. _and: .. _or: .. _not:
.. index:: pair: Conditional; expression pair: Boolean; operation
.. productionlist:: python-grammar
or_test: and_test | or_test "or" and_test
and_test: not_test | and_test "and" not_test
not_test: comparison | "not" not_test
In the context of Boolean operations, and also when expressions are used by
control flow statements, the following values are interpreted as false:
False, None, 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 :meth:~object.__bool__ method.
.. index:: pair: operator; not
The operator :keyword:not yields True if its argument is false, False
otherwise.
.. index:: pair: operator; and
The expression x and y first evaluates x; if x is false, its value is
returned; otherwise, y is evaluated and the resulting value is returned.
.. index:: pair: operator; or
The expression x or y first evaluates x; if x is true, its value is
returned; otherwise, y is evaluated and the resulting value is returned.
Note that neither :keyword:and nor :keyword:or restrict the value and type
they return to False and True, but rather return the last evaluated
argument. This is sometimes useful, e.g., if s is a string that should be
replaced by a default value if it is empty, the expression s or 'foo' yields
the desired value. Because :keyword:not has to create a new value, it
returns a boolean value regardless of the type of its argument
(for example, not 'foo' produces False rather than ''.)
.. index:: single: := (colon equals) single: assignment expression single: walrus operator single: named expression pair: assignment; expression
.. _assignment-expressions:
.. productionlist:: python-grammar
assignment_expression: [identifier ":="] expression
An assignment expression (sometimes also called a "named expression" or
"walrus") assigns an :token:~python-grammar:expression to an
:token:~python-grammar:identifier, while also returning the value of the
:token:~python-grammar:expression.
One common use case is when handling matched regular expressions:
.. code-block:: python
if matching := pattern.search(data): do_something(matching)
Or, when processing a file stream in chunks:
.. code-block:: python
while chunk := file.read(9000): process(chunk)
Assignment expressions must be surrounded by parentheses when
used as expression statements and when used as sub-expressions in
slicing, conditional, lambda,
keyword-argument, and comprehension-if expressions and
in assert, with, and assignment statements.
In all other places where they can be used, parentheses are not required,
including in if and while statements.
.. versionadded:: 3.8
See :pep:572 for more details about assignment expressions.
.. _if_expr:
.. index:: pair: conditional; expression pair: ternary; operator single: if; conditional expression single: else; conditional expression
.. productionlist:: python-grammar
conditional_expression: or_test ["if" or_test "else" expression]
expression: conditional_expression | lambda_expr
A conditional expression (sometimes called a "ternary operator") is an alternative to the if-else statement. As it is an expression, it returns a value and can appear as a sub-expression.
The expression x if C else y 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 :pep:308 for more details about conditional expressions.
.. _lambdas: .. _lambda:
.. index:: pair: lambda; expression pair: lambda; form pair: anonymous; function single: : (colon); lambda expression
.. productionlist:: python-grammar
lambda_expr: "lambda" [parameter_list] ":" expression
Lambda expressions (sometimes called lambda forms) are used to create anonymous
functions. The expression lambda parameters: expression yields a function
object. The unnamed object behaves like a function object defined with:
.. code-block:: none
def <lambda>(parameters): return expression
See section :ref:function for the syntax of parameter lists. Note that
functions created with lambda expressions cannot contain statements or
annotations.
.. _exprlists:
.. index:: pair: expression; list single: , (comma); expression list
.. productionlist:: python-grammar
starred_expression: "" or_expr | expression
flexible_expression: assignment_expression | starred_expression
flexible_expression_list: flexible_expression ("," flexible_expression) [","]
starred_expression_list: starred_expression ("," starred_expression)* [","]
expression_list: expression ("," expression)* [","]
yield_list: expression_list | starred_expression "," [starred_expression_list]
.. index:: pair: object; tuple
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.
.. index:: pair: iterable; unpacking single: * (asterisk); in expression lists
An asterisk * denotes :dfn:iterable unpacking. Its operand must be
an :term:iterable. 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.
.. versionadded:: 3.5
Iterable unpacking in expression lists, originally proposed by :pep:448.
.. versionadded:: 3.11
Any item in an expression list may be starred. See :pep:646.
.. index:: pair: trailing; comma
A trailing comma is required only to create a one-item tuple,
such as 1,; 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:
().)
.. _evalorder:
.. index:: pair: evaluation; order
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::
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
.. _operator-summary:
.. index:: pair: operator; precedence
The following table summarizes the operator precedence in Python, from highest precedence (most binding) to lowest precedence (least 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 and conditional expressions, which group 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
:ref:comparisons section.
+-----------------------------------------------+-------------------------------------+
| Operator | Description |
+===============================================+=====================================+
| (expressions...), | Binding or parenthesized |
| | expression, |
| [expressions...], | list display, |
| {key: value...}, | dictionary display, |
| {expressions...} | set display |
+-----------------------------------------------+-------------------------------------+
| x[index], x[index:index] | Subscription (including slicing), |
| x(arguments...), x.attribute | call, attribute reference |
+-----------------------------------------------+-------------------------------------+
| :keyword:await x <await> | Await expression |
+-----------------------------------------------+-------------------------------------+
| ** | Exponentiation [#]_ |
+-----------------------------------------------+-------------------------------------+
| +x, -x, ~x | Positive, negative, bitwise NOT |
+-----------------------------------------------+-------------------------------------+
| *, @, /, //, % | Multiplication, matrix |
| | multiplication, division, floor |
| | division, remainder [#]_ |
+-----------------------------------------------+-------------------------------------+
| +, - | Addition and subtraction |
+-----------------------------------------------+-------------------------------------+
| <<, >> | Shifts |
+-----------------------------------------------+-------------------------------------+
| & | Bitwise AND |
+-----------------------------------------------+-------------------------------------+
| ^ | Bitwise XOR |
+-----------------------------------------------+-------------------------------------+
| | | Bitwise OR |
+-----------------------------------------------+-------------------------------------+
| :keyword:in, :keyword:not in, | Comparisons, including membership |
| :keyword:is, :keyword:is not, <, | tests and identity tests |
| <=, >, >=, !=, == | |
+-----------------------------------------------+-------------------------------------+
| :keyword:not x <not> | Boolean NOT |
+-----------------------------------------------+-------------------------------------+
| :keyword:and | Boolean AND |
+-----------------------------------------------+-------------------------------------+
| :keyword:or | Boolean OR |
+-----------------------------------------------+-------------------------------------+
| :keyword:if <if_expr> -- :keyword:!else | Conditional expression |
+-----------------------------------------------+-------------------------------------+
| :keyword:lambda | Lambda expression |
+-----------------------------------------------+-------------------------------------+
| := | Assignment expression |
+-----------------------------------------------+-------------------------------------+
.. rubric:: Footnotes
.. [#] While abs(x%y) < abs(y) 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 -1e-100 % 1e100 have the same sign as 1e100, the computed result is -1e-100 + 1e100, which is numerically exactly equal to 1e100. The function
:func:math.fmod returns a result whose sign matches the sign of the
first argument instead, and so returns -1e-100 in this case. Which approach
is more appropriate depends on the application.
.. [#] If x is very close to an exact integer multiple of y, it's possible for
x//y to be one larger than (x-x%y)//y due to rounding. In such
cases, Python returns the latter result, in order to preserve that
divmod(x,y)[0] * y + x % y be very close to x.
.. [#] The Unicode standard distinguishes between :dfn:code points
(e.g. U+0041) and :dfn: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 :dfn:precomposed character at code position U+00C7, or as a
sequence of a :dfn:base character at code position U+0043 (LATIN CAPITAL
LETTER C), followed by a :dfn: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,
"\u00C7" == "\u0043\u0327" is False, 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 :func:unicodedata.normalize.
.. [#] Due to automatic garbage-collection, free lists, and the dynamic nature of
descriptors, you may notice seemingly unusual behaviour in certain uses of
the :keyword:is operator, like those involving comparisons between instance
methods, or constants. Check their documentation for more info.
.. [#] The power operator ** binds less tightly than an arithmetic or
bitwise unary operator on its right, that is, 2**-1 is 0.5.
.. [#] The % operator is also used for string formatting; the same
precedence applies.