.. _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:: ! pair: statement; if pair: keyword; elif pair: 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:: ! pair: statement; while pair: 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:: pair: statement; break pair: 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:: ! pair: statement; for pair: keyword; in pair: keyword; else pair: target; list pair: loop; statement pair: 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_expression_list ":" suite : ["else" ":" suite]

The :token:~python-grammar:starred_expression_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:: pair: statement; break pair: 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:: pair: built-in function; 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:: ! pair: statement; try pair: keyword; except pair: keyword; finally pair: keyword; else pair: 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.

.. versionchanged:: 3.14 Support for optionally dropping grouping parentheses when using multiple exception types. See :pep:758.

.. _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, the expression must evaluate to an exception type or a tuple of exception types. Parentheses can be dropped if multiple exception types are provided and the as clause is not used. The raised exception matches an :keyword:!except clause whose expression evaluates to the class or a :term:non-virtual base class <abstract base class> of the exception object, or to a tuple that contains such a class.

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:: pair: module; sys pair: object; traceback

Before an :keyword:!except clause's suite is executed, the exception is stored in the :mod:sys module, where it can be accessed from within the body of the :keyword:!except clause by calling :func:sys.exception. When leaving an exception handler, the exception stored in the :mod:sys module is reset to its previous value::

print(sys.exception()) None try: ... raise TypeError ... except: ... print(repr(sys.exception())) ... try: ... raise ValueError ... except: ... print(repr(sys.exception())) ... print(repr(sys.exception())) ... TypeError() ValueError() TypeError() print(sys.exception()) None

.. index:: pair: keyword; except_star

.. _except_star:

:keyword:!except* clause

The :keyword:!except* clause(s) specify one or more handlers for groups of exceptions (:exc:BaseExceptionGroup instances). A :keyword:try statement can have either :keyword:except or :keyword:!except* clauses, but not both. The exception type for matching is mandatory in the case of :keyword:!except*, so except*: is a syntax error. The type is interpreted as in the case of :keyword:!except, but matching is performed on the exceptions contained in the group that is being handled. An :exc:TypeError is raised if a matching type is a subclass of :exc:!BaseExceptionGroup, because that would have ambiguous semantics.

When an exception group is raised in the try block, each :keyword:!except* clause splits (see :meth:~BaseExceptionGroup.split) it into the subgroups of matching and non-matching exceptions. If the matching subgroup is not empty, it becomes the handled exception (the value returned from :func:sys.exception) and assigned to the target of the :keyword:!except* clause (if there is one). Then, the body of the :keyword:!except* clause executes. If the non-matching subgroup is not empty, it is processed by the next :keyword:!except* in the same manner. This continues until all exceptions in the group have been matched, or the last :keyword:!except* clause has run.

After all :keyword:!except* clauses execute, the group of unhandled exceptions is merged with any exceptions that were raised or re-raised from within :keyword:!except* clauses. This merged exception group propagates on.::

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 "<doctest default[0]>", line 2, in <module> | raise ExceptionGroup("eg", | [ValueError(1), TypeError(2), OSError(3), OSError(4)]) | ExceptionGroup: eg (1 sub-exception) +-+---------------- 1 ---------------- | ValueError: 1 +------------------------------------

If the exception raised from the :keyword:try block 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. This ensures that the type of the target e is consistently :exc:BaseExceptionGroup::

try: ... raise BlockingIOError ... except* BlockingIOError as e: ... print(repr(e)) ... ExceptionGroup('', (BlockingIOError(),))

:keyword:break, :keyword:continue and :keyword:return cannot appear in an :keyword:!except* clause.

.. index:: pair: keyword; else pair: statement; return pair: statement; break pair: 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:: pair: 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 <except_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. For example, this function returns 42.

.. code-block::

def f(): try: 1/0 finally: return 42

The exception information is not available to the program during execution of the :keyword:!finally clause.

.. index:: pair: statement; return pair: statement; break pair: 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. The following function returns 'finally'.

.. code-block::

def foo(): try: return 'try' finally: return '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.

.. versionchanged:: 3.14 The compiler emits a :exc:SyntaxWarning when a :keyword:return, :keyword:break or :keyword:continue appears in a :keyword:!finally block (see :pep:765).

.. _with: .. _as:

The :keyword:!with statement

.. index:: ! pair: statement; with pair: 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:~object.__enter__ is loaded for later use.

#. The context manager's :meth:~object.__exit__ is loaded for later use.

#. The context manager's :meth:~object.__enter__ method is invoked.

#. If a target was included in the :keyword:with statement, the return value from :meth:~object.__enter__ is assigned to it.

.. note::

  The :keyword:`with` statement guarantees that if the :meth:`~object.__enter__`
  method returns without an error, then :meth:`~object.__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:~object.__exit__ method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to :meth:~object.__exit__. Otherwise, three :const:None arguments are supplied.

If the suite was exited due to an exception, and the return value from the :meth:~object.__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:~object.__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 = manager.__enter__
exit = manager.__exit__
value = enter()
hit_except = False

try:
    TARGET = value
    SUITE
except:
    hit_except = True
    if not exit(*sys.exc_info()):
        raise
finally:
    if not hit_except:
        exit(None, None, None)

except that implicit :ref:special method lookup <special-lookup> is used for :meth:~object.__enter__ and :meth:~object.__exit__.

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: .. _case:

The :keyword:!match statement

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

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

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 and :ref:t-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:~object.__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:~object.__missing__ or :meth:~object.__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:: pair: statement; def pair: function; definition pair: function; name pair: name; binding pair: object; user-defined function pair: 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 [type_params] "(" [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: "" [star_parameter] ("," defparameter) ["," [parameter_star_kwargs]] : | "" ("," defparameter)+ ["," [parameter_star_kwargs]] : | parameter_star_kwargs parameter_star_kwargs: "**" parameter [","] parameter: identifier [":" expression] star_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.

A list of :ref:type parameters <type-params> may be given in square brackets between the function's name and the opening parenthesis for its parameter list. This indicates to static type checkers that the function is generic. At runtime, the type parameters can be retrieved from the function's :attr:~function.__type_params__ attribute. See :ref:generic-functions for more.

.. versionchanged:: 3.12 Type parameter lists are new in Python 3.12.

.. 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. (As a special case, parameters of the form *identifier may have an annotation ": *expression".) 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. See :ref:annotations for more information on annotations.

.. versionchanged:: 3.11 Parameters of the form "*identifier" may have an annotation ": *expression". See :pep:646.

.. 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.

:pep:318 - Decorators for Functions and Methods Function and method decorators were introduced. Class decorators were introduced in :pep:3129.

.. _class:

Class definitions

.. index:: pair: object; class pair: 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 [type_params] [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

There may be one or more base classes; see :ref:multiple-inheritance below for more information.

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 :attr:~type.__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.

A list of :ref:type parameters <type-params> may be given in square brackets immediately after the class's name. This indicates to static type checkers that the class is generic. At runtime, the type parameters can be retrieved from the class's :attr:~type.__type_params__ attribute. See :ref:generic-classes for more.

.. versionchanged:: 3.12 Type parameter lists are new in Python 3.12.

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.

.. _multiple-inheritance:

Multiple inheritance

Python classes may have multiple base classes, a technique known as multiple inheritance. The base classes are specified in the class definition by listing them in parentheses after the class name, separated by commas. For example, the following class definition:

.. doctest::

class A: pass class B: pass class C(A, B): pass

defines a class C that inherits from classes A and B.

The :term:method resolution order (MRO) is the order in which base classes are searched when looking up an attribute on a class. See :ref:python_2.3_mro for a description of how Python determines the MRO for a class.

Multiple inheritance is not always allowed. Attempting to define a class with multiple inheritance will raise an error if one of the bases does not allow subclassing, if a consistent MRO cannot be created, if no valid metaclass can be determined, or if there is an instance layout conflict. We'll discuss each of these in turn.

First, all base classes must allow subclassing. While most classes allow subclassing, some built-in classes do not, such as :class:bool:

.. doctest::

class SubBool(bool): # TypeError ... pass Traceback (most recent call last): ... TypeError: type 'bool' is not an acceptable base type

In the resolved MRO of a class, the class's bases appear in the order they were specified in the class's bases list. Additionally, the MRO always lists a child class before any of its bases. A class definition will fail if it is impossible to resolve a consistent MRO that satisfies these rules from the list of bases provided:

.. doctest::

class Base: pass class Child(Base): pass class Grandchild(Base, Child): pass # TypeError Traceback (most recent call last): ... TypeError: Cannot create a consistent method resolution order (MRO) for bases Base, Child

In the MRO of Grandchild, Base must appear before Child because it is first in the base class list, but it must also appear after Child because it is a parent of Child. This is a contradiction, so the class cannot be defined.

If some of the bases have a custom :term:metaclass, the metaclass of the resulting class is chosen among the metaclasses of the bases and the explicitly specified metaclass of the child class. It must be a metaclass that is a subclass of all other candidate metaclasses. If no such metaclass exists among the candidates, the class cannot be created, as explained in :ref:metaclass-determination.

Finally, the instance layouts of the bases must be compatible. This means that it must be possible to compute a solid base for the class. Exactly which classes are solid bases depends on the Python implementation.

.. impl-detail::

In CPython, a class is a solid base if it has a nonempty :attr:~object.__slots__ definition. Many but not all classes defined in C are also solid bases, including most builtins (such as :class:int or :class:BaseException) but excluding most concrete :class:Exception classes. Generally, a C class is a solid base if its underlying struct is different in size from its base class.

Every class has a solid base. :class:object, the base class, has itself as its solid base. If there is a single base, the child class's solid base is that class if it is a solid base, or else the base class's solid base. If there are multiple bases, we first find the solid base for each base class to produce a list of candidate solid bases. If there is a unique solid base that is a subclass of all others, then that class is the solid base. Otherwise, class creation fails.

Example:

.. doctest::

class Solid1: ... slots = ("solid1",)

class Solid2: ... slots = ("solid2",)

class SolidChild(Solid1): ... slots = ("solid_child",)

class C1: # solid base is object ... pass

OK: solid bases are Solid1 and object, and Solid1 is a subclass of object.

class C2(Solid1, C1): # solid base is Solid1 ... pass

OK: solid bases are SolidChild and Solid1, and SolidChild is a subclass of Solid1.

class C3(SolidChild, Solid1): # solid base is SolidChild ... pass

Error: solid bases are Solid1 and Solid2, but neither is a subclass of the other.

class C4(Solid1, Solid2): # error: no single solid base ... pass Traceback (most recent call last): ... TypeError: multiple bases have instance lay-out conflict

.. _async:

Coroutines

.. versionadded:: 3.5

.. index:: pair: statement; async def .. _async def:

Coroutine function definition

.. productionlist:: python-grammar async_funcdef: [decorators] "async" "def" funcname "(" [parameter_list] ")" : ["->" expression] ":" suite

.. index:: pair: keyword; async pair: 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:: pair: 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).__aiter__()
running = True

while running:
    try:
        TARGET = await iter.__anext__()
    except StopAsyncIteration:
        running = False
    else:
        SUITE
else:
    SUITE2

except that implicit :ref:special method lookup <special-lookup> is used for :meth:~object.__aiter__ and :meth:~object.__anext__.

It is a :exc:SyntaxError to use an async for statement outside the body of a coroutine function.

.. index:: pair: 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 = manager.__aenter__
aexit = manager.__aexit__
value = await aenter()
hit_except = False

try:
    TARGET = value
    SUITE
except:
    hit_except = True
    if not await aexit(*sys.exc_info()):
        raise
finally:
    if not hit_except:
        await aexit(None, None, None)

except that implicit :ref:special method lookup <special-lookup> is used for :meth:~object.__aenter__ and :meth:~object.__aexit__.

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.

.. _type-params:

Type parameter lists

.. versionadded:: 3.12

.. versionchanged:: 3.13 Support for default values was added (see :pep:696).

.. index:: single: type parameters

.. productionlist:: python-grammar type_params: "[" type_param ("," type_param)* "]" type_param: typevar | typevartuple | paramspec typevar: identifier (":" expression)? ("=" expression)? typevartuple: "*" identifier ("=" expression)? paramspec: "**" identifier ("=" expression)?

:ref:Functions <def> (including :ref:coroutines <async def>), :ref:classes <class> and :ref:type aliases <type> may contain a type parameter list::

def max[T](args: list[T]) -> T: ...

async def amax[T](args: list[T]) -> T: ...

class Bag[T]: def iter(self) -> Iterator[T]: ...

   def add(self, arg: T) -> None:
       ...

type ListOrSet[T] = list[T] | set[T]

Semantically, this indicates that the function, class, or type alias is generic over a type variable. This information is primarily used by static type checkers, and at runtime, generic objects behave much like their non-generic counterparts.

Type parameters are declared in square brackets ([]) immediately after the name of the function, class, or type alias. The type parameters are accessible within the scope of the generic object, but not elsewhere. Thus, after a declaration def func[T](): pass, the name T is not available in the module scope. Below, the semantics of generic objects are described with more precision. The scope of type parameters is modeled with a special function (technically, an :ref:annotation scope <annotation-scopes>) that wraps the creation of the generic object.

Generic functions, classes, and type aliases have a :attr:~definition.__type_params__ attribute listing their type parameters.

Type parameters come in three kinds:

• :data:typing.TypeVar, introduced by a plain name (e.g., T). Semantically, this represents a single type to a type checker.
• :data:typing.TypeVarTuple, introduced by a name prefixed with a single asterisk (e.g., *Ts). Semantically, this stands for a tuple of any number of types.
• :data:typing.ParamSpec, introduced by a name prefixed with two asterisks (e.g., **P). Semantically, this stands for the parameters of a callable.

:data:typing.TypeVar declarations can define bounds and constraints with a colon (:) followed by an expression. A single expression after the colon indicates a bound (e.g. T: int). Semantically, this means that the :data:!typing.TypeVar can only represent types that are a subtype of this bound. A parenthesized tuple of expressions after the colon indicates a set of constraints (e.g. T: (str, bytes)). Each member of the tuple should be a type (again, this is not enforced at runtime). Constrained type variables can only take on one of the types in the list of constraints.

For :data:!typing.TypeVar\ s declared using the type parameter list syntax, the bound and constraints are not evaluated when the generic object is created, but only when the value is explicitly accessed through the attributes __bound__ and __constraints__. To accomplish this, the bounds or constraints are evaluated in a separate :ref:annotation scope <annotation-scopes>.

:data:typing.TypeVarTuple\ s and :data:typing.ParamSpec\ s cannot have bounds or constraints.

All three flavors of type parameters can also have a default value, which is used when the type parameter is not explicitly provided. This is added by appending a single equals sign (=) followed by an expression. Like the bounds and constraints of type variables, the default value is not evaluated when the object is created, but only when the type parameter's __default__ attribute is accessed. To this end, the default value is evaluated in a separate :ref:annotation scope <annotation-scopes>. If no default value is specified for a type parameter, the __default__ attribute is set to the special sentinel object :data:typing.NoDefault.

The following example indicates the full set of allowed type parameter declarations::

def overly_generic[ SimpleTypeVar, TypeVarWithDefault = int, TypeVarWithBound: int, TypeVarWithConstraints: (str, bytes), *SimpleTypeVarTuple = (int, float), **SimpleParamSpec = (str, bytearray), ]( a: SimpleTypeVar, b: TypeVarWithDefault, c: TypeVarWithBound, d: Callable[SimpleParamSpec, TypeVarWithConstraints], *e: SimpleTypeVarTuple, ): ...

.. _generic-functions:

Generic functions

Generic functions are declared as follows::

def func[T](arg: T): ...

This syntax is equivalent to::

annotation-def TYPE_PARAMS_OF_func(): T = typing.TypeVar("T") def func(arg: T): ... func.type_params = (T,) return func func = TYPE_PARAMS_OF_func()

Here annotation-def indicates an :ref:annotation scope <annotation-scopes>, which is not actually bound to any name at runtime. (One other liberty is taken in the translation: the syntax does not go through attribute access on the :mod:typing module, but creates an instance of :data:typing.TypeVar directly.)

The annotations of generic functions are evaluated within the annotation scope used for declaring the type parameters, but the function's defaults and decorators are not.

The following example illustrates the scoping rules for these cases, as well as for additional flavors of type parameters::

@decorator def func[T: int, *Ts, **P](*args: *Ts, arg: Callable[P, T] = some_default): ...

Except for the :ref:lazy evaluation <lazy-evaluation> of the :class:~typing.TypeVar bound, this is equivalent to::

DEFAULT_OF_arg = some_default

annotation-def TYPE_PARAMS_OF_func():

   annotation-def BOUND_OF_T():
       return int
   # In reality, BOUND_OF_T() is evaluated only on demand.
   T = typing.TypeVar("T", bound=BOUND_OF_T())

   Ts = typing.TypeVarTuple("Ts")
   P = typing.ParamSpec("P")

   def func(*args: *Ts, arg: Callable[P, T] = DEFAULT_OF_arg):
       ...

   func.__type_params__ = (T, Ts, P)
   return func

func = decorator(TYPE_PARAMS_OF_func())

The capitalized names like DEFAULT_OF_arg are not actually bound at runtime.

.. _generic-classes:

Generic classes

Generic classes are declared as follows::

class Bag[T]: ...

This syntax is equivalent to::

annotation-def TYPE_PARAMS_OF_Bag(): T = typing.TypeVar("T") class Bag(typing.Generic[T]): type_params = (T,) ... return Bag Bag = TYPE_PARAMS_OF_Bag()

Here again annotation-def (not a real keyword) indicates an :ref:annotation scope <annotation-scopes>, and the name TYPE_PARAMS_OF_Bag is not actually bound at runtime.

Generic classes implicitly inherit from :data:typing.Generic. The base classes and keyword arguments of generic classes are evaluated within the type scope for the type parameters, and decorators are evaluated outside that scope. This is illustrated by this example::

@decorator class Bag(Base[T], arg=T): ...

This is equivalent to::

annotation-def TYPE_PARAMS_OF_Bag(): T = typing.TypeVar("T") class Bag(Base[T], typing.Generic[T], arg=T): type_params = (T,) ... return Bag Bag = decorator(TYPE_PARAMS_OF_Bag())

.. _generic-type-aliases:

Generic type aliases

The :keyword:type statement can also be used to create a generic type alias::

type ListOrSet[T] = list[T] | set[T]

Except for the :ref:lazy evaluation <lazy-evaluation> of the value, this is equivalent to::

annotation-def TYPE_PARAMS_OF_ListOrSet(): T = typing.TypeVar("T")

   annotation-def VALUE_OF_ListOrSet():
       return list[T] | set[T]
   # In reality, the value is lazily evaluated
   return typing.TypeAliasType("ListOrSet", VALUE_OF_ListOrSet(), type_params=(T,))

ListOrSet = TYPE_PARAMS_OF_ListOrSet()

Here, annotation-def (not a real keyword) indicates an :ref:annotation scope <annotation-scopes>. The capitalized names like TYPE_PARAMS_OF_ListOrSet are not actually bound at runtime.

.. _annotations:

Annotations

.. versionchanged:: 3.14 Annotations are now lazily evaluated by default.

Variables and function parameters may carry :term:annotations <annotation>, created by adding a colon after the name, followed by an expression::

x: annotation = 1 def f(param: annotation): ...

Functions may also carry a return annotation following an arrow::

def f() -> annotation: ...

Annotations are conventionally used for :term:type hints <type hint>, but this is not enforced by the language, and in general annotations may contain arbitrary expressions. The presence of annotations does not change the runtime semantics of the code, except if some mechanism is used that introspects and uses the annotations (such as :mod:dataclasses or :func:functools.singledispatch).

By default, annotations are lazily evaluated in an :ref:annotation scope <annotation-scopes>. This means that they are not evaluated when the code containing the annotation is evaluated. Instead, the interpreter saves information that can be used to evaluate the annotation later if requested. The :mod:annotationlib module provides tools for evaluating annotations.

If the :ref:future statement <future> from __future__ import annotations is present, all annotations are instead stored as strings::

from future import annotations def f(param: annotation): ... f.annotations {'param': 'annotation'}

This future statement will be deprecated and removed in a future version of Python, but not before Python 3.13 reaches its end of life (see :pep:749). When it is used, introspection tools like :func:annotationlib.get_annotations and :func:typing.get_type_hints are less likely to be able to resolve annotations at runtime.

.. 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) :c:macro: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) :c:macro: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 :attr:~function.__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 :attr:~type.__doc__ item and therefore the class's :term:docstring.