compound_stmts.rst

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.

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.

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.

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 function :func:`range` returns an iterator of integers suitable to emulate the effect of Pascal's for i := a to b do; e.g., list(range(3)) returns the list [0, 1, 2].

.. versionchanged:: 3.11
   Starred elements are now allowed in the expression list.

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`.

: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. [1]

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

: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 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.

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

: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

: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.

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:

1. The context expression (the expression given in the :token:`~python-grammar:with_item`) is evaluated to obtain a context manager.

2. The context manager's :meth:`__enter__` is loaded for later use.

3. The context manager's :meth:`__exit__` is loaded for later use.

4. The context manager's :meth:`__enter__` method is invoked.

5. 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 6 below.

6. The suite is executed.

7. 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.

The :keyword:`!match` statement

.. index::
   ! statement: match
   ! keyword: case
   ! single: pattern matching
   keyword: if
   keyword: as
   pair: match; case
   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:

1. 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>`.

2. 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.

3. 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:

1. 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.
2. 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 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

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

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

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

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

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, guards, and case blocks.

In simple terms, _ will always succeed.

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

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

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:

1. If the subject value is not a sequence [2], the sequence pattern fails.

2. If the subject value is an instance of str, bytes or bytearray the sequence pattern fails.

3. The subsequent steps depend on whether the sequence pattern is fixed or variable-length.

   If the sequence pattern is fixed-length:

   1. If the length of the subject sequence is not equal to the number of subpatterns, the sequence pattern fails
   2. 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:

   1. If the length of the subject sequence is less than the number of non-star subpatterns, the sequence pattern fails.
   2. The leading non-star subpatterns are matched to their corresponding items as for fixed-length sequences.
   3. 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.
   4. 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

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:

1. If the subject value is not a mapping [3],the mapping pattern fails.
2. 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.
3. 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

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:

1. If name_or_attr is not an instance of the builtin :class:`type` , raise :exc:`TypeError`.

2. If the subject value is not an instance of name_or_attr (tested via :func:`isinstance`), the class pattern fails.

3. 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:

   1. 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.
   2. 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:

   1. 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`
      
      

   2. 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 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. [4]

.. 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 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. [5] 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`.

Coroutines

.. versionadded:: 3.5

.. index:: statement: 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

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

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.

Footnotes

[1]	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.

[2]

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.

[3]

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.

[4]	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`.

[5]	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`.