.. _compound:
*******************
Compound statements
*******************
.. index:: pair: compound; statement
Compound statements contain (groups of) other statements; they affect or control
the execution of those other statements in some way. In general, compound
statements span multiple lines, although in simple incarnations a whole compound
statement may be contained in one line.
The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement
traditional control flow constructs. :keyword:`try` specifies exception
handlers and/or cleanup code for a group of statements, while the
:keyword:`with` statement allows the execution of initialization and
finalization code around a block of code. Function and class definitions are
also syntactically compound statements.
.. index::
single: clause
single: suite
single: ; (semicolon)
A compound statement consists of one or more 'clauses.' A clause consists of a
header and a 'suite.' The clause headers of a particular compound statement are
all at the same indentation level. Each clause header begins with a uniquely
identifying keyword and ends with a colon. A suite is a group of statements
controlled by a clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's colon, or it
can be one or more indented statements on subsequent lines. Only the latter
form of a suite can contain nested compound statements; the following is illegal,
mostly because it wouldn't be clear to which :keyword:`if` clause a following
:keyword:`else` clause would belong::
if test1: if test2: print(x)
Also note that the semicolon binds tighter than the colon in this context, so
that in the following example, either all or none of the :func:`print` calls are
executed::
if x < y < z: print(x); print(y); print(z)
Summarizing:
.. productionlist:: python-grammar
compound_stmt: `if_stmt`
: | `while_stmt`
: | `for_stmt`
: | `try_stmt`
: | `with_stmt`
: | `match_stmt`
: | `funcdef`
: | `classdef`
: | `async_with_stmt`
: | `async_for_stmt`
: | `async_funcdef`
suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
statement: `stmt_list` NEWLINE | `compound_stmt`
stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
.. index::
single: NEWLINE token
single: DEDENT token
pair: dangling; else
Note that statements always end in a ``NEWLINE`` possibly followed by a
``DEDENT``. Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities (the
'dangling :keyword:`else`' problem is solved in Python by requiring nested
:keyword:`if` statements to be indented).
The formatting of the grammar rules in the following sections places each clause
on a separate line for clarity.
.. _if:
.. _elif:
.. _else:
The :keyword:`!if` statement
============================
.. index::
! statement: if
keyword: elif
keyword: else
single: : (colon); compound statement
The :keyword:`if` statement is used for conditional execution:
.. productionlist:: python-grammar
if_stmt: "if" `assignment_expression` ":" `suite`
: ("elif" `assignment_expression` ":" `suite`)*
: ["else" ":" `suite`]
It selects exactly one of the suites by evaluating the expressions one by one
until one is found to be true (see section :ref:`booleans` for the definition of
true and false); then that suite is executed (and no other part of the
:keyword:`if` statement is executed or evaluated). If all expressions are
false, the suite of the :keyword:`else` clause, if present, is executed.
.. _while:
The :keyword:`!while` statement
===============================
.. index::
! statement: while
keyword: else
pair: loop; statement
single: : (colon); compound statement
The :keyword:`while` statement is used for repeated execution as long as an
expression is true:
.. productionlist:: python-grammar
while_stmt: "while" `assignment_expression` ":" `suite`
: ["else" ":" `suite`]
This repeatedly tests the expression and, if it is true, executes the first
suite; if the expression is false (which may be the first time it is tested) the
suite of the :keyword:`!else` clause, if present, is executed and the loop
terminates.
.. index::
statement: break
statement: continue
A :keyword:`break` statement executed in the first suite terminates the loop
without executing the :keyword:`!else` clause's suite. A :keyword:`continue`
statement executed in the first suite skips the rest of the suite and goes back
to testing the expression.
.. _for:
The :keyword:`!for` statement
=============================
.. index::
! statement: for
keyword: in
keyword: else
pair: target; list
pair: loop; statement
object: sequence
single: : (colon); compound statement
The :keyword:`for` statement is used to iterate over the elements of a sequence
(such as a string, tuple or list) or other iterable object:
.. productionlist:: python-grammar
for_stmt: "for" `target_list` "in" `starred_list` ":" `suite`
: ["else" ":" `suite`]
The ``starred_list`` expression is evaluated once; it should yield an
:term:`iterable` object. An :term:`iterator` is created for that iterable.
The first item provided
by the iterator is then assigned to the target list using the standard
rules for assignments (see :ref:`assignment`), and the suite is executed. This
repeats for each item provided by the iterator. When the iterator is exhausted,
the suite in the :keyword:`!else` clause,
if present, is executed, and the loop terminates.
.. index::
statement: break
statement: continue
A :keyword:`break` statement executed in the first suite terminates the loop
without executing the :keyword:`!else` clause's suite. A :keyword:`continue`
statement executed in the first suite skips the rest of the suite and continues
with the next item, or with the :keyword:`!else` clause if there is no next
item.
The for-loop makes assignments to the variables in the target list.
This overwrites all previous assignments to those variables including
those made in the suite of the for-loop::
for i in range(10):
print(i)
i = 5 # this will not affect the for-loop
# because i will be overwritten with the next
# index in the range
.. index::
builtin: range
Names in the target list are not deleted when the loop is finished, but if the
sequence is empty, they will not have been assigned to at all by the loop. Hint:
the built-in type :func:`range` represents immutable arithmetic sequences of integers.
For instance, iterating ``range(3)`` successively yields 0, 1, and then 2.
.. versionchanged:: 3.11
Starred elements are now allowed in the expression list.
.. _try:
The :keyword:`!try` statement
=============================
.. index::
! statement: try
keyword: except
keyword: finally
keyword: else
keyword: as
single: : (colon); compound statement
The :keyword:`!try` statement specifies exception handlers and/or cleanup code
for a group of statements:
.. productionlist:: python-grammar
try_stmt: `try1_stmt` | `try2_stmt` | `try3_stmt`
try1_stmt: "try" ":" `suite`
: ("except" [`expression` ["as" `identifier`]] ":" `suite`)+
: ["else" ":" `suite`]
: ["finally" ":" `suite`]
try2_stmt: "try" ":" `suite`
: ("except" "*" `expression` ["as" `identifier`] ":" `suite`)+
: ["else" ":" `suite`]
: ["finally" ":" `suite`]
try3_stmt: "try" ":" `suite`
: "finally" ":" `suite`
Additional information on exceptions can be found in section :ref:`exceptions`,
and information on using the :keyword:`raise` statement to generate exceptions
may be found in section :ref:`raise`.
.. _except:
:keyword:`!except` clause
-------------------------
The :keyword:`!except` clause(s) specify one or more exception handlers. When no
exception occurs in the :keyword:`try` clause, no exception handler is executed.
When an exception occurs in the :keyword:`!try` suite, a search for an exception
handler is started. This search inspects the :keyword:`!except` clauses in turn
until one is found that matches the exception.
An expression-less :keyword:`!except` clause, if present, must be last;
it matches any exception.
For an :keyword:`!except` clause with an expression,
that expression is evaluated, and the clause matches the exception
if the resulting object is "compatible" with the exception. An object is
compatible with an exception if the object is the class or a
:term:`non-virtual base class <abstract base class>` of the exception object,
or a tuple containing an item that is the class or a non-virtual base class
of the exception object.
If no :keyword:`!except` clause matches the exception,
the search for an exception handler
continues in the surrounding code and on the invocation stack. [#]_
If the evaluation of an expression
in the header of an :keyword:`!except` clause raises an exception,
the original search for a handler is canceled and a search starts for
the new exception in the surrounding code and on the call stack (it is treated
as if the entire :keyword:`try` statement raised the exception).
.. index:: single: as; except clause
When a matching :keyword:`!except` clause is found,
the exception is assigned to the target
specified after the :keyword:`!as` keyword in that :keyword:`!except` clause,
if present, and the :keyword:`!except` clause's suite is executed.
All :keyword:`!except` clauses must have an executable block.
When the end of this block is reached, execution continues
normally after the entire :keyword:`try` statement.
(This means that if two nested handlers exist for the same exception,
and the exception occurs in the :keyword:`!try` clause of the inner handler,
the outer handler will not handle the exception.)
When an exception has been assigned using ``as target``, it is cleared at the
end of the :keyword:`!except` clause. This is as if ::
except E as N:
foo
was translated to ::
except E as N:
try:
foo
finally:
del N
This means the exception must be assigned to a different name to be able to
refer to it after the :keyword:`!except` clause.
Exceptions are cleared because with the
traceback attached to them, they form a reference cycle with the stack frame,
keeping all locals in that frame alive until the next garbage collection occurs.
.. index::
module: sys
object: traceback
Before an :keyword:`!except` clause's suite is executed,
details about the exception are
stored in the :mod:`sys` module and can be accessed via :func:`sys.exc_info`.
:func:`sys.exc_info` returns a 3-tuple consisting of the exception class, the
exception instance and a traceback object (see section :ref:`types`) identifying
the point in the program where the exception occurred. The details about the
exception accessed via :func:`sys.exc_info` are restored to their previous values
when leaving an exception handler::
>>> print(sys.exc_info())
(None, None, None)
>>> try:
... raise TypeError
... except:
... print(sys.exc_info())
... try:
... raise ValueError
... except:
... print(sys.exc_info())
... print(sys.exc_info())
...
(<class 'TypeError'>, TypeError(), <traceback object at 0x10efad080>)
(<class 'ValueError'>, ValueError(), <traceback object at 0x10efad040>)
(<class 'TypeError'>, TypeError(), <traceback object at 0x10efad080>)
>>> print(sys.exc_info())
(None, None, None)
.. index::
keyword: except_star
.. _except_star:
:keyword:`!except*` clause
--------------------------
The :keyword:`!except*` clause(s) are used for handling
:exc:`ExceptionGroup`\s. The exception type for matching is interpreted as in
the case of :keyword:`except`, but in the case of exception groups we can have
partial matches when the type matches some of the exceptions in the group.
This means that multiple :keyword:`!except*` clauses can execute,
each handling part of the exception group.
Each clause executes at most once and handles an exception group
of all matching exceptions. Each exception in the group is handled by at most
one :keyword:`!except*` clause, the first that matches it. ::
>>> try:
... raise ExceptionGroup("eg",
... [ValueError(1), TypeError(2), OSError(3), OSError(4)])
... except* TypeError as e:
... print(f'caught {type(e)} with nested {e.exceptions}')
... except* OSError as e:
... print(f'caught {type(e)} with nested {e.exceptions}')
...
caught <class 'ExceptionGroup'> with nested (TypeError(2),)
caught <class 'ExceptionGroup'> with nested (OSError(3), OSError(4))
+ Exception Group Traceback (most recent call last):
| File "<stdin>", line 2, in <module>
| ExceptionGroup: eg
+-+---------------- 1 ----------------
| ValueError: 1
+------------------------------------
Any remaining exceptions that were not handled by any :keyword:`!except*`
clause are re-raised at the end, combined into an exception group along with
all exceptions that were raised from within :keyword:`!except*` clauses.
If the raised exception is not an exception group and its type matches
one of the :keyword:`!except*` clauses, it is caught and wrapped by an
exception group with an empty message string. ::
>>> try:
... raise BlockingIOError
... except* BlockingIOError as e:
... print(repr(e))
...
ExceptionGroup('', (BlockingIOError()))
An :keyword:`!except*` clause must have a matching type,
and this type cannot be a subclass of :exc:`BaseExceptionGroup`.
It is not possible to mix :keyword:`except` and :keyword:`!except*`
in the same :keyword:`try`.
:keyword:`break`, :keyword:`continue` and :keyword:`return`
cannot appear in an :keyword:`!except*` clause.
.. index::
keyword: else
statement: return
statement: break
statement: continue
.. _except_else:
:keyword:`!else` clause
-----------------------
The optional :keyword:`!else` clause is executed if the control flow leaves the
:keyword:`try` suite, no exception was raised, and no :keyword:`return`,
:keyword:`continue`, or :keyword:`break` statement was executed. Exceptions in
the :keyword:`!else` clause are not handled by the preceding :keyword:`except`
clauses.
.. index:: keyword: finally
.. _finally:
:keyword:`!finally` clause
--------------------------
If :keyword:`!finally` is present, it specifies a 'cleanup' handler. The
:keyword:`try` clause is executed, including any :keyword:`except` and
:keyword:`else` clauses. If an exception occurs in any of the clauses and is
not handled, the exception is temporarily saved. The :keyword:`!finally` clause
is executed. If there is a saved exception it is re-raised at the end of the
:keyword:`!finally` clause. If the :keyword:`!finally` clause raises another
exception, the saved exception is set as the context of the new exception.
If the :keyword:`!finally` clause executes a :keyword:`return`, :keyword:`break`
or :keyword:`continue` statement, the saved exception is discarded::
>>> def f():
... try:
... 1/0
... finally:
... return 42
...
>>> f()
42
The exception information is not available to the program during execution of
the :keyword:`!finally` clause.
.. index::
statement: return
statement: break
statement: continue
When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
executed in the :keyword:`try` suite of a :keyword:`!try`...\ :keyword:`!finally`
statement, the :keyword:`!finally` clause is also executed 'on the way out.'
The return value of a function is determined by the last :keyword:`return`
statement executed. Since the :keyword:`!finally` clause always executes, a
:keyword:`!return` statement executed in the :keyword:`!finally` clause will
always be the last one executed::
>>> def foo():
... try:
... return 'try'
... finally:
... return 'finally'
...
>>> foo()
'finally'
.. versionchanged:: 3.8
Prior to Python 3.8, a :keyword:`continue` statement was illegal in the
:keyword:`!finally` clause due to a problem with the implementation.
.. _with:
.. _as:
The :keyword:`!with` statement
==============================
.. index::
! statement: with
keyword: as
single: as; with statement
single: , (comma); with statement
single: : (colon); compound statement
The :keyword:`with` statement is used to wrap the execution of a block with
methods defined by a context manager (see section :ref:`context-managers`).
This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally`
usage patterns to be encapsulated for convenient reuse.
.. productionlist:: python-grammar
with_stmt: "with" ( "(" `with_stmt_contents` ","? ")" | `with_stmt_contents` ) ":" `suite`
with_stmt_contents: `with_item` ("," `with_item`)*
with_item: `expression` ["as" `target`]
The execution of the :keyword:`with` statement with one "item" proceeds as follows:
#. The context expression (the expression given in the
:token:`~python-grammar:with_item`) is evaluated to obtain a context manager.
#. The context manager's :meth:`__enter__` is loaded for later use.
#. The context manager's :meth:`__exit__` is loaded for later use.
#. The context manager's :meth:`__enter__` method is invoked.
#. If a target was included in the :keyword:`with` statement, the return value
from :meth:`__enter__` is assigned to it.
.. note::
The :keyword:`with` statement guarantees that if the :meth:`__enter__`
method returns without an error, then :meth:`__exit__` will always be
called. Thus, if an error occurs during the assignment to the target list,
it will be treated the same as an error occurring within the suite would
be. See step 7 below.
#. The suite is executed.
#. The context manager's :meth:`__exit__` method is invoked. If an exception
caused the suite to be exited, its type, value, and traceback are passed as
arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
supplied.
If the suite was exited due to an exception, and the return value from the
:meth:`__exit__` method was false, the exception is reraised. If the return
value was true, the exception is suppressed, and execution continues with the
statement following the :keyword:`with` statement.
If the suite was exited for any reason other than an exception, the return
value from :meth:`__exit__` is ignored, and execution proceeds at the normal
location for the kind of exit that was taken.
The following code::
with EXPRESSION as TARGET:
SUITE
is semantically equivalent to::
manager = (EXPRESSION)
enter = type(manager).__enter__
exit = type(manager).__exit__
value = enter(manager)
hit_except = False
try:
TARGET = value
SUITE
except:
hit_except = True
if not exit(manager, *sys.exc_info()):
raise
finally:
if not hit_except:
exit(manager, None, None, None)
With more than one item, the context managers are processed as if multiple
:keyword:`with` statements were nested::
with A() as a, B() as b:
SUITE
is semantically equivalent to::
with A() as a:
with B() as b:
SUITE
You can also write multi-item context managers in multiple lines if
the items are surrounded by parentheses. For example::
with (
A() as a,
B() as b,
):
SUITE
.. versionchanged:: 3.1
Support for multiple context expressions.
.. versionchanged:: 3.10
Support for using grouping parentheses to break the statement in multiple lines.
.. seealso::
:pep:`343` - The "with" statement
The specification, background, and examples for the Python :keyword:`with`
statement.
.. _match:
The :keyword:`!match` statement
===============================
.. index::
! statement: match
! keyword: case
! single: pattern matching
keyword: if
keyword: as
pair: match; case
single: as; match statement
single: : (colon); compound statement
.. versionadded:: 3.10
The match statement is used for pattern matching. Syntax:
.. productionlist:: python-grammar
match_stmt: 'match' `subject_expr` ":" NEWLINE INDENT `case_block`+ DEDENT
subject_expr: `star_named_expression` "," `star_named_expressions`?
: | `named_expression`
case_block: 'case' `patterns` [`guard`] ":" `block`
.. note::
This section uses single quotes to denote
:ref:`soft keywords <soft-keywords>`.
Pattern matching takes a pattern as input (following ``case``) and a subject
value (following ``match``). The pattern (which may contain subpatterns) is
matched against the subject value. The outcomes are:
* A match success or failure (also termed a pattern success or failure).
* Possible binding of matched values to a name. The prerequisites for this are
further discussed below.
The ``match`` and ``case`` keywords are :ref:`soft keywords <soft-keywords>`.
.. seealso::
* :pep:`634` -- Structural Pattern Matching: Specification
* :pep:`636` -- Structural Pattern Matching: Tutorial
Overview
--------
Here's an overview of the logical flow of a match statement:
#. The subject expression ``subject_expr`` is evaluated and a resulting subject
value obtained. If the subject expression contains a comma, a tuple is
constructed using :ref:`the standard rules <typesseq-tuple>`.
#. Each pattern in a ``case_block`` is attempted to match with the subject value. The
specific rules for success or failure are described below. The match attempt can also
bind some or all of the standalone names within the pattern. The precise
pattern binding rules vary per pattern type and are
specified below. **Name bindings made during a successful pattern match
outlive the executed block and can be used after the match statement**.
.. note::
During failed pattern matches, some subpatterns may succeed. Do not
rely on bindings being made for a failed match. Conversely, do not
rely on variables remaining unchanged after a failed match. The exact
behavior is dependent on implementation and may vary. This is an
intentional decision made to allow different implementations to add
optimizations.
#. If the pattern succeeds, the corresponding guard (if present) is evaluated. In
this case all name bindings are guaranteed to have happened.
* If the guard evaluates as true or is missing, the ``block`` inside
``case_block`` is executed.
* Otherwise, the next ``case_block`` is attempted as described above.
* If there are no further case blocks, the match statement is completed.
.. note::
Users should generally never rely on a pattern being evaluated. Depending on
implementation, the interpreter may cache values or use other optimizations
which skip repeated evaluations.
A sample match statement::
>>> flag = False
>>> match (100, 200):
... case (100, 300): # Mismatch: 200 != 300
... print('Case 1')
... case (100, 200) if flag: # Successful match, but guard fails
... print('Case 2')
... case (100, y): # Matches and binds y to 200
... print(f'Case 3, y: {y}')
... case _: # Pattern not attempted
... print('Case 4, I match anything!')
...
Case 3, y: 200
In this case, ``if flag`` is a guard. Read more about that in the next section.
Guards
------
.. index:: ! guard
.. productionlist:: python-grammar
guard: "if" `named_expression`
A ``guard`` (which is part of the ``case``) must succeed for code inside
the ``case`` block to execute. It takes the form: :keyword:`if` followed by an
expression.
The logical flow of a ``case`` block with a ``guard`` follows:
#. Check that the pattern in the ``case`` block succeeded. If the pattern
failed, the ``guard`` is not evaluated and the next ``case`` block is
checked.
#. If the pattern succeeded, evaluate the ``guard``.
* If the ``guard`` condition evaluates as true, the case block is
selected.
* If the ``guard`` condition evaluates as false, the case block is not
selected.
* If the ``guard`` raises an exception during evaluation, the exception
bubbles up.
Guards are allowed to have side effects as they are expressions. Guard
evaluation must proceed from the first to the last case block, one at a time,
skipping case blocks whose pattern(s) don't all succeed. (I.e.,
guard evaluation must happen in order.) Guard evaluation must stop once a case
block is selected.
.. _irrefutable_case:
Irrefutable Case Blocks
-----------------------
.. index:: irrefutable case block, case block
An irrefutable case block is a match-all case block. A match statement may have
at most one irrefutable case block, and it must be last.
A case block is considered irrefutable if it has no guard and its pattern is
irrefutable. A pattern is considered irrefutable if we can prove from its
syntax alone that it will always succeed. Only the following patterns are
irrefutable:
* :ref:`as-patterns` whose left-hand side is irrefutable
* :ref:`or-patterns` containing at least one irrefutable pattern
* :ref:`capture-patterns`
* :ref:`wildcard-patterns`
* parenthesized irrefutable patterns
Patterns
--------
.. index::
single: ! patterns
single: AS pattern, OR pattern, capture pattern, wildcard pattern
.. note::
This section uses grammar notations beyond standard EBNF:
* the notation ``SEP.RULE+`` is shorthand for ``RULE (SEP RULE)*``
* the notation ``!RULE`` is shorthand for a negative lookahead assertion
The top-level syntax for ``patterns`` is:
.. productionlist:: python-grammar
patterns: `open_sequence_pattern` | `pattern`
pattern: `as_pattern` | `or_pattern`
closed_pattern: | `literal_pattern`
: | `capture_pattern`
: | `wildcard_pattern`
: | `value_pattern`
: | `group_pattern`
: | `sequence_pattern`
: | `mapping_pattern`
: | `class_pattern`
The descriptions below will include a description "in simple terms" of what a pattern
does for illustration purposes (credits to Raymond Hettinger for a document that
inspired most of the descriptions). Note that these descriptions are purely for
illustration purposes and **may not** reflect
the underlying implementation. Furthermore, they do not cover all valid forms.
.. _or-patterns:
OR Patterns
^^^^^^^^^^^
An OR pattern is two or more patterns separated by vertical
bars ``|``. Syntax:
.. productionlist:: python-grammar
or_pattern: "|".`closed_pattern`+
Only the final subpattern may be :ref:`irrefutable <irrefutable_case>`, and each
subpattern must bind the same set of names to avoid ambiguity.
An OR pattern matches each of its subpatterns in turn to the subject value,
until one succeeds. The OR pattern is then considered successful. Otherwise,
if none of the subpatterns succeed, the OR pattern fails.
In simple terms, ``P1 | P2 | ...`` will try to match ``P1``, if it fails it will try to
match ``P2``, succeeding immediately if any succeeds, failing otherwise.
.. _as-patterns:
AS Patterns
^^^^^^^^^^^
An AS pattern matches an OR pattern on the left of the :keyword:`as`
keyword against a subject. Syntax:
.. productionlist:: python-grammar
as_pattern: `or_pattern` "as" `capture_pattern`
If the OR pattern fails, the AS pattern fails. Otherwise, the AS pattern binds
the subject to the name on the right of the as keyword and succeeds.
``capture_pattern`` cannot be a a ``_``.
In simple terms ``P as NAME`` will match with ``P``, and on success it will
set ``NAME = <subject>``.
.. _literal-patterns:
Literal Patterns
^^^^^^^^^^^^^^^^
A literal pattern corresponds to most
:ref:`literals <literals>` in Python. Syntax:
.. productionlist:: python-grammar
literal_pattern: `signed_number`
: | `signed_number` "+" NUMBER
: | `signed_number` "-" NUMBER
: | `strings`
: | "None"
: | "True"
: | "False"
: | `signed_number`: NUMBER | "-" NUMBER
The rule ``strings`` and the token ``NUMBER`` are defined in the
:doc:`standard Python grammar <./grammar>`. Triple-quoted strings are
supported. Raw strings and byte strings are supported. :ref:`f-strings` are
not supported.
The forms ``signed_number '+' NUMBER`` and ``signed_number '-' NUMBER`` are
for expressing :ref:`complex numbers <imaginary>`; they require a real number
on the left and an imaginary number on the right. E.g. ``3 + 4j``.
In simple terms, ``LITERAL`` will succeed only if ``<subject> == LITERAL``. For
the singletons ``None``, ``True`` and ``False``, the :keyword:`is` operator is used.
.. _capture-patterns:
Capture Patterns
^^^^^^^^^^^^^^^^
A capture pattern binds the subject value to a name.
Syntax:
.. productionlist:: python-grammar
capture_pattern: !'_' NAME
A single underscore ``_`` is not a capture pattern (this is what ``!'_'``
expresses). It is instead treated as a
:token:`~python-grammar:wildcard_pattern`.
In a given pattern, a given name can only be bound once. E.g.
``case x, x: ...`` is invalid while ``case [x] | x: ...`` is allowed.
Capture patterns always succeed. The binding follows scoping rules
established by the assignment expression operator in :pep:`572`; the
name becomes a local variable in the closest containing function scope unless
there's an applicable :keyword:`global` or :keyword:`nonlocal` statement.
In simple terms ``NAME`` will always succeed and it will set ``NAME = <subject>``.
.. _wildcard-patterns:
Wildcard Patterns
^^^^^^^^^^^^^^^^^
A wildcard pattern always succeeds (matches anything)
and binds no name. Syntax:
.. productionlist:: python-grammar
wildcard_pattern: '_'
``_`` is a :ref:`soft keyword <soft-keywords>` within any pattern,
but only within patterns. It is an identifier, as usual, even within
``match`` subject expressions, ``guard``\ s, and ``case`` blocks.
In simple terms, ``_`` will always succeed.
.. _value-patterns:
Value Patterns
^^^^^^^^^^^^^^
A value pattern represents a named value in Python.
Syntax:
.. productionlist:: python-grammar
value_pattern: `attr`
attr: `name_or_attr` "." NAME
name_or_attr: `attr` | NAME
The dotted name in the pattern is looked up using standard Python
:ref:`name resolution rules <resolve_names>`. The pattern succeeds if the
value found compares equal to the subject value (using the ``==`` equality
operator).
In simple terms ``NAME1.NAME2`` will succeed only if ``<subject> == NAME1.NAME2``
.. note::
If the same value occurs multiple times in the same match statement, the
interpreter may cache the first value found and reuse it rather than repeat
the same lookup. This cache is strictly tied to a given execution of a
given match statement.
.. _group-patterns:
Group Patterns
^^^^^^^^^^^^^^
A group pattern allows users to add parentheses around patterns to
emphasize the intended grouping. Otherwise, it has no additional syntax.
Syntax:
.. productionlist:: python-grammar
group_pattern: "(" `pattern` ")"
In simple terms ``(P)`` has the same effect as ``P``.
.. _sequence-patterns:
Sequence Patterns
^^^^^^^^^^^^^^^^^
A sequence pattern contains several subpatterns to be matched against sequence elements.
The syntax is similar to the unpacking of a list or tuple.
.. productionlist:: python-grammar
sequence_pattern: "[" [`maybe_sequence_pattern`] "]"
: | "(" [`open_sequence_pattern`] ")"
open_sequence_pattern: `maybe_star_pattern` "," [`maybe_sequence_pattern`]
maybe_sequence_pattern: ",".`maybe_star_pattern`+ ","?
maybe_star_pattern: `star_pattern` | `pattern`
star_pattern: "*" (`capture_pattern` | `wildcard_pattern`)
There is no difference if parentheses or square brackets
are used for sequence patterns (i.e. ``(...)`` vs ``[...]`` ).
.. note::
A single pattern enclosed in parentheses without a trailing comma
(e.g. ``(3 | 4)``) is a :ref:`group pattern <group-patterns>`.
While a single pattern enclosed in square brackets (e.g. ``[3 | 4]``) is
still a sequence pattern.
At most one star subpattern may be in a sequence pattern. The star subpattern
may occur in any position. If no star subpattern is present, the sequence
pattern is a fixed-length sequence pattern; otherwise it is a variable-length
sequence pattern.
The following is the logical flow for matching a sequence pattern against a
subject value:
#. If the subject value is not a sequence [#]_, the sequence pattern
fails.
#. If the subject value is an instance of ``str``, ``bytes`` or ``bytearray``
the sequence pattern fails.
#. The subsequent steps depend on whether the sequence pattern is fixed or
variable-length.
If the sequence pattern is fixed-length:
#. If the length of the subject sequence is not equal to the number of
subpatterns, the sequence pattern fails
#. Subpatterns in the sequence pattern are matched to their corresponding
items in the subject sequence from left to right. Matching stops as soon
as a subpattern fails. If all subpatterns succeed in matching their
corresponding item, the sequence pattern succeeds.
Otherwise, if the sequence pattern is variable-length:
#. If the length of the subject sequence is less than the number of non-star
subpatterns, the sequence pattern fails.
#. The leading non-star subpatterns are matched to their corresponding items
as for fixed-length sequences.
#. If the previous step succeeds, the star subpattern matches a list formed
of the remaining subject items, excluding the remaining items
corresponding to non-star subpatterns following the star subpattern.
#. Remaining non-star subpatterns are matched to their corresponding subject
items, as for a fixed-length sequence.
.. note:: The length of the subject sequence is obtained via
:func:`len` (i.e. via the :meth:`__len__` protocol). This length may be
cached by the interpreter in a similar manner as
:ref:`value patterns <value-patterns>`.
In simple terms ``[P1, P2, P3,`` ... ``, P<N>]`` matches only if all the following
happens:
* check ``<subject>`` is a sequence
* ``len(subject) == <N>``
* ``P1`` matches ``<subject>[0]`` (note that this match can also bind names)
* ``P2`` matches ``<subject>[1]`` (note that this match can also bind names)
* ... and so on for the corresponding pattern/element.
.. _mapping-patterns:
Mapping Patterns
^^^^^^^^^^^^^^^^
A mapping pattern contains one or more key-value patterns. The syntax is
similar to the construction of a dictionary.
Syntax:
.. productionlist:: python-grammar
mapping_pattern: "{" [`items_pattern`] "}"
items_pattern: ",".`key_value_pattern`+ ","?
key_value_pattern: (`literal_pattern` | `value_pattern`) ":" `pattern`
: | `double_star_pattern`
double_star_pattern: "**" `capture_pattern`
At most one double star pattern may be in a mapping pattern. The double star
pattern must be the last subpattern in the mapping pattern.
Duplicate keys in mapping patterns are disallowed. Duplicate literal keys will
raise a :exc:`SyntaxError`. Two keys that otherwise have the same value will
raise a :exc:`ValueError` at runtime.
The following is the logical flow for matching a mapping pattern against a
subject value:
#. If the subject value is not a mapping [#]_,the mapping pattern fails.
#. If every key given in the mapping pattern is present in the subject mapping,
and the pattern for each key matches the corresponding item of the subject
mapping, the mapping pattern succeeds.
#. If duplicate keys are detected in the mapping pattern, the pattern is
considered invalid. A :exc:`SyntaxError` is raised for duplicate literal
values; or a :exc:`ValueError` for named keys of the same value.
.. note:: Key-value pairs are matched using the two-argument form of the mapping
subject's ``get()`` method. Matched key-value pairs must already be present
in the mapping, and not created on-the-fly via :meth:`__missing__` or
:meth:`__getitem__`.
In simple terms ``{KEY1: P1, KEY2: P2, ... }`` matches only if all the following
happens:
* check ``<subject>`` is a mapping
* ``KEY1 in <subject>``
* ``P1`` matches ``<subject>[KEY1]``
* ... and so on for the corresponding KEY/pattern pair.
.. _class-patterns:
Class Patterns
^^^^^^^^^^^^^^
A class pattern represents a class and its positional and keyword arguments
(if any). Syntax:
.. productionlist:: python-grammar
class_pattern: `name_or_attr` "(" [`pattern_arguments` ","?] ")"
pattern_arguments: `positional_patterns` ["," `keyword_patterns`]
: | `keyword_patterns`
positional_patterns: ",".`pattern`+
keyword_patterns: ",".`keyword_pattern`+
keyword_pattern: NAME "=" `pattern`
The same keyword should not be repeated in class patterns.
The following is the logical flow for matching a class pattern against a
subject value:
#. If ``name_or_attr`` is not an instance of the builtin :class:`type` , raise
:exc:`TypeError`.
#. If the subject value is not an instance of ``name_or_attr`` (tested via
:func:`isinstance`), the class pattern fails.
#. If no pattern arguments are present, the pattern succeeds. Otherwise,
the subsequent steps depend on whether keyword or positional argument patterns
are present.
For a number of built-in types (specified below), a single positional
subpattern is accepted which will match the entire subject; for these types
keyword patterns also work as for other types.
If only keyword patterns are present, they are processed as follows,
one by one:
I. The keyword is looked up as an attribute on the subject.
* If this raises an exception other than :exc:`AttributeError`, the
exception bubbles up.
* If this raises :exc:`AttributeError`, the class pattern has failed.
* Else, the subpattern associated with the keyword pattern is matched
against the subject's attribute value. If this fails, the class
pattern fails; if this succeeds, the match proceeds to the next keyword.
II. If all keyword patterns succeed, the class pattern succeeds.
If any positional patterns are present, they are converted to keyword
patterns using the :data:`~object.__match_args__` attribute on the class
``name_or_attr`` before matching:
I. The equivalent of ``getattr(cls, "__match_args__", ())`` is called.
* If this raises an exception, the exception bubbles up.
* If the returned value is not a tuple, the conversion fails and
:exc:`TypeError` is raised.
* If there are more positional patterns than ``len(cls.__match_args__)``,
:exc:`TypeError` is raised.
* Otherwise, positional pattern ``i`` is converted to a keyword pattern
using ``__match_args__[i]`` as the keyword. ``__match_args__[i]`` must
be a string; if not :exc:`TypeError` is raised.
* If there are duplicate keywords, :exc:`TypeError` is raised.
.. seealso:: :ref:`class-pattern-matching`
II. Once all positional patterns have been converted to keyword patterns,
the match proceeds as if there were only keyword patterns.
For the following built-in types the handling of positional subpatterns is
different:
* :class:`bool`
* :class:`bytearray`
* :class:`bytes`
* :class:`dict`
* :class:`float`
* :class:`frozenset`
* :class:`int`
* :class:`list`
* :class:`set`
* :class:`str`
* :class:`tuple`
These classes accept a single positional argument, and the pattern there is matched
against the whole object rather than an attribute. For example ``int(0|1)`` matches
the value ``0``, but not the value ``0.0``.
In simple terms ``CLS(P1, attr=P2)`` matches only if the following happens:
* ``isinstance(<subject>, CLS)``
* convert ``P1`` to a keyword pattern using ``CLS.__match_args__``
* For each keyword argument ``attr=P2``:
* ``hasattr(<subject>, "attr")``
* ``P2`` matches ``<subject>.attr``
* ... and so on for the corresponding keyword argument/pattern pair.
.. seealso::
* :pep:`634` -- Structural Pattern Matching: Specification
* :pep:`636` -- Structural Pattern Matching: Tutorial
.. index::
single: parameter; function definition
.. _function:
.. _def:
Function definitions
====================
.. index::
statement: def
pair: function; definition
pair: function; name
pair: name; binding
object: user-defined function
object: function
pair: function; name
pair: name; binding
single: () (parentheses); function definition
single: , (comma); parameter list
single: : (colon); compound statement
A function definition defines a user-defined function object (see section
:ref:`types`):
.. productionlist:: python-grammar
funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")"
: ["->" `expression`] ":" `suite`
decorators: `decorator`+
decorator: "@" `assignment_expression` NEWLINE
parameter_list: `defparameter` ("," `defparameter`)* "," "/" ["," [`parameter_list_no_posonly`]]
: | `parameter_list_no_posonly`
parameter_list_no_posonly: `defparameter` ("," `defparameter`)* ["," [`parameter_list_starargs`]]
: | `parameter_list_starargs`
parameter_list_starargs: "*" [`parameter`] ("," `defparameter`)* ["," ["**" `parameter` [","]]]
: | "**" `parameter` [","]
parameter: `identifier` [":" `expression`]
defparameter: `parameter` ["=" `expression`]
funcname: `identifier`
A function definition is an executable statement. Its execution binds the
function name in the current local namespace to a function object (a wrapper
around the executable code for the function). This function object contains a
reference to the current global namespace as the global namespace to be used
when the function is called.
The function definition does not execute the function body; this gets executed
only when the function is called. [#]_
.. index::
single: @ (at); function definition
A function definition may be wrapped by one or more :term:`decorator` expressions.
Decorator expressions are evaluated when the function is defined, in the scope
that contains the function definition. The result must be a callable, which is
invoked with the function object as the only argument. The returned value is
bound to the function name instead of the function object. Multiple decorators
are applied in nested fashion. For example, the following code ::
@f1(arg)
@f2
def func(): pass
is roughly equivalent to ::
def func(): pass
func = f1(arg)(f2(func))
except that the original function is not temporarily bound to the name ``func``.
.. versionchanged:: 3.9
Functions may be decorated with any valid
:token:`~python-grammar:assignment_expression`. Previously, the grammar was
much more restrictive; see :pep:`614` for details.
.. index::
triple: default; parameter; value
single: argument; function definition
single: = (equals); function definition
When one or more :term:`parameters <parameter>` have the form *parameter* ``=``
*expression*, the function is said to have "default parameter values." For a
parameter with a default value, the corresponding :term:`argument` may be
omitted from a call, in which
case the parameter's default value is substituted. If a parameter has a default
value, all following parameters up until the "``*``" must also have a default
value --- this is a syntactic restriction that is not expressed by the grammar.
**Default parameter values are evaluated from left to right when the function
definition is executed.** This means that the expression is evaluated once, when
the function is defined, and that the same "pre-computed" value is used for each
call. This is especially important to understand when a default parameter value is a
mutable object, such as a list or a dictionary: if the function modifies the
object (e.g. by appending an item to a list), the default parameter value is in effect
modified. This is generally not what was intended. A way around this is to use
``None`` as the default, and explicitly test for it in the body of the function,
e.g.::
def whats_on_the_telly(penguin=None):
if penguin is None:
penguin = []
penguin.append("property of the zoo")
return penguin
.. index::
single: / (slash); function definition
single: * (asterisk); function definition
single: **; function definition
Function call semantics are described in more detail in section :ref:`calls`. A
function call always assigns values to all parameters mentioned in the parameter
list, either from positional arguments, from keyword arguments, or from default
values. If the form "``*identifier``" is present, it is initialized to a tuple
receiving any excess positional parameters, defaulting to the empty tuple.
If the form "``**identifier``" is present, it is initialized to a new
ordered mapping receiving any excess keyword arguments, defaulting to a
new empty mapping of the same type. Parameters after "``*``" or
"``*identifier``" are keyword-only parameters and may only be passed
by keyword arguments. Parameters before "``/``" are positional-only parameters
and may only be passed by positional arguments.
.. versionchanged:: 3.8
The ``/`` function parameter syntax may be used to indicate positional-only
parameters. See :pep:`570` for details.
.. index::
pair: function; annotations
single: ->; function annotations
single: : (colon); function annotations
Parameters may have an :term:`annotation <function annotation>` of the form "``: expression``"
following the parameter name. Any parameter may have an annotation, even those of the form
``*identifier`` or ``**identifier``. Functions may have "return" annotation of
the form "``-> expression``" after the parameter list. These annotations can be
any valid Python expression. The presence of annotations does not change the
semantics of a function. The annotation values are available as values of
a dictionary keyed by the parameters' names in the :attr:`__annotations__`
attribute of the function object. If the ``annotations`` import from
:mod:`__future__` is used, annotations are preserved as strings at runtime which
enables postponed evaluation. Otherwise, they are evaluated when the function
definition is executed. In this case annotations may be evaluated in
a different order than they appear in the source code.
.. index:: pair: lambda; expression
It is also possible to create anonymous functions (functions not bound to a
name), for immediate use in expressions. This uses lambda expressions, described in
section :ref:`lambda`. Note that the lambda expression is merely a shorthand for a
simplified function definition; a function defined in a ":keyword:`def`"
statement can be passed around or assigned to another name just like a function
defined by a lambda expression. The ":keyword:`!def`" form is actually more powerful
since it allows the execution of multiple statements and annotations.
**Programmer's note:** Functions are first-class objects. A "``def``" statement
executed inside a function definition defines a local function that can be
returned or passed around. Free variables used in the nested function can
access the local variables of the function containing the def. See section
:ref:`naming` for details.
.. seealso::
:pep:`3107` - Function Annotations
The original specification for function annotations.
:pep:`484` - Type Hints
Definition of a standard meaning for annotations: type hints.
:pep:`526` - Syntax for Variable Annotations
Ability to type hint variable declarations, including class
variables and instance variables
:pep:`563` - Postponed Evaluation of Annotations
Support for forward references within annotations by preserving
annotations in a string form at runtime instead of eager evaluation.
.. _class:
Class definitions
=================
.. index::
object: class
statement: class
pair: class; definition
pair: class; name
pair: name; binding
pair: execution; frame
single: inheritance
single: docstring
single: () (parentheses); class definition
single: , (comma); expression list
single: : (colon); compound statement
A class definition defines a class object (see section :ref:`types`):
.. productionlist:: python-grammar
classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite`
inheritance: "(" [`argument_list`] ")"
classname: `identifier`
A class definition is an executable statement. The inheritance list usually
gives a list of base classes (see :ref:`metaclasses` for more advanced uses), so
each item in the list should evaluate to a class object which allows
subclassing. Classes without an inheritance list inherit, by default, from the
base class :class:`object`; hence, ::
class Foo:
pass
is equivalent to ::
class Foo(object):
pass
The class's suite is then executed in a new execution frame (see :ref:`naming`),
using a newly created local namespace and the original global namespace.
(Usually, the suite contains mostly function definitions.) When the class's
suite finishes execution, its execution frame is discarded but its local
namespace is saved. [#]_ A class object is then created using the inheritance
list for the base classes and the saved local namespace for the attribute
dictionary. The class name is bound to this class object in the original local
namespace.
The order in which attributes are defined in the class body is preserved
in the new class's ``__dict__``. Note that this is reliable only right
after the class is created and only for classes that were defined using
the definition syntax.
Class creation can be customized heavily using :ref:`metaclasses <metaclasses>`.
.. index::
single: @ (at); class definition
Classes can also be decorated: just like when decorating functions, ::
@f1(arg)
@f2
class Foo: pass
is roughly equivalent to ::
class Foo: pass
Foo = f1(arg)(f2(Foo))
The evaluation rules for the decorator expressions are the same as for function
decorators. The result is then bound to the class name.
.. versionchanged:: 3.9
Classes may be decorated with any valid
:token:`~python-grammar:assignment_expression`. Previously, the grammar was
much more restrictive; see :pep:`614` for details.
**Programmer's note:** Variables defined in the class definition are class
attributes; they are shared by instances. Instance attributes can be set in a
method with ``self.name = value``. Both class and instance attributes are
accessible through the notation "``self.name``", and an instance attribute hides
a class attribute with the same name when accessed in this way. Class
attributes can be used as defaults for instance attributes, but using mutable
values there can lead to unexpected results. :ref:`Descriptors <descriptors>`
can be used to create instance variables with different implementation details.
.. seealso::
:pep:`3115` - Metaclasses in Python 3000
The proposal that changed the declaration of metaclasses to the current
syntax, and the semantics for how classes with metaclasses are
constructed.
:pep:`3129` - Class Decorators
The proposal that added class decorators. Function and method decorators
were introduced in :pep:`318`.
.. _async:
Coroutines
==========
.. versionadded:: 3.5
.. index:: statement: async def
.. _`async def`:
Coroutine function definition
-----------------------------
.. productionlist:: python-grammar
async_funcdef: [`decorators`] "async" "def" `funcname` "(" [`parameter_list`] ")"
: ["->" `expression`] ":" `suite`
.. index::
keyword: async
keyword: await
Execution of Python coroutines can be suspended and resumed at many points
(see :term:`coroutine`). :keyword:`await` expressions, :keyword:`async for` and
:keyword:`async with` can only be used in the body of a coroutine function.
Functions defined with ``async def`` syntax are always coroutine functions,
even if they do not contain ``await`` or ``async`` keywords.
It is a :exc:`SyntaxError` to use a ``yield from`` expression inside the body
of a coroutine function.
An example of a coroutine function::
async def func(param1, param2):
do_stuff()
await some_coroutine()
.. versionchanged:: 3.7
``await`` and ``async`` are now keywords; previously they were only
treated as such inside the body of a coroutine function.
.. index:: statement: async for
.. _`async for`:
The :keyword:`!async for` statement
-----------------------------------
.. productionlist:: python-grammar
async_for_stmt: "async" `for_stmt`
An :term:`asynchronous iterable` provides an ``__aiter__`` method that directly
returns an :term:`asynchronous iterator`, which can call asynchronous code in
its ``__anext__`` method.
The ``async for`` statement allows convenient iteration over asynchronous
iterables.
The following code::
async for TARGET in ITER:
SUITE
else:
SUITE2
Is semantically equivalent to::
iter = (ITER)
iter = type(iter).__aiter__(iter)
running = True
while running:
try:
TARGET = await type(iter).__anext__(iter)
except StopAsyncIteration:
running = False
else:
SUITE
else:
SUITE2
See also :meth:`~object.__aiter__` and :meth:`~object.__anext__` for details.
It is a :exc:`SyntaxError` to use an ``async for`` statement outside the
body of a coroutine function.
.. index:: statement: async with
.. _`async with`:
The :keyword:`!async with` statement
------------------------------------
.. productionlist:: python-grammar
async_with_stmt: "async" `with_stmt`
An :term:`asynchronous context manager` is a :term:`context manager` that is
able to suspend execution in its *enter* and *exit* methods.
The following code::
async with EXPRESSION as TARGET:
SUITE
is semantically equivalent to::
manager = (EXPRESSION)
aenter = type(manager).__aenter__
aexit = type(manager).__aexit__
value = await aenter(manager)
hit_except = False
try:
TARGET = value
SUITE
except:
hit_except = True
if not await aexit(manager, *sys.exc_info()):
raise
finally:
if not hit_except:
await aexit(manager, None, None, None)
See also :meth:`~object.__aenter__` and :meth:`~object.__aexit__` for details.
It is a :exc:`SyntaxError` to use an ``async with`` statement outside the
body of a coroutine function.
.. seealso::
:pep:`492` - Coroutines with async and await syntax
The proposal that made coroutines a proper standalone concept in Python,
and added supporting syntax.
.. rubric:: Footnotes
.. [#] The exception is propagated to the invocation stack unless
there is a :keyword:`finally` clause which happens to raise another
exception. That new exception causes the old one to be lost.
.. [#] In pattern matching, a sequence is defined as one of the following:
* a class that inherits from :class:`collections.abc.Sequence`
* a Python class that has been registered as :class:`collections.abc.Sequence`
* a builtin class that has its (CPython) :data:`Py_TPFLAGS_SEQUENCE` bit set
* a class that inherits from any of the above
The following standard library classes are sequences:
* :class:`array.array`
* :class:`collections.deque`
* :class:`list`
* :class:`memoryview`
* :class:`range`
* :class:`tuple`
.. note:: Subject values of type ``str``, ``bytes``, and ``bytearray``
do not match sequence patterns.
.. [#] In pattern matching, a mapping is defined as one of the following:
* a class that inherits from :class:`collections.abc.Mapping`
* a Python class that has been registered as :class:`collections.abc.Mapping`
* a builtin class that has its (CPython) :data:`Py_TPFLAGS_MAPPING` bit set
* a class that inherits from any of the above
The standard library classes :class:`dict` and :class:`types.MappingProxyType`
are mappings.
.. [#] A string literal appearing as the first statement in the function body is
transformed into the function's ``__doc__`` attribute and therefore the
function's :term:`docstring`.
.. [#] A string literal appearing as the first statement in the class body is
transformed into the namespace's ``__doc__`` item and therefore the class's
:term:`docstring`.
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