 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]

A function definition may be wrapped by one or more *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".

Changed in version 3.9: Functions may be decorated with any valid
"assignment_expression". Previously, the grammar was much more
restrictive; see **PEP 614** for details.

A list of type parameters 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
"__type_params__" attribute. See Generic functions for more.

Changed in version 3.12: Type parameter lists are new in Python 3.12.

When one or more *parameters* have the form *parameter* "="
*expression*, the function is said to have “default parameter values.”
For a parameter with a default value, the corresponding *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

Function call semantics are described in more detail in section 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.

Changed in version 3.8: The "/" function parameter syntax may be used
to indicate positional-only parameters. See **PEP 570** for details.

Parameters may have an *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.  The annotation values are
available as values of a dictionary keyed by the parameters’ names in
the "__annotations__" attribute of the function object.  If the
"annotations" import from "__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.

Changed in version 3.11: Parameters of the form “"*identifier"” may
have an annotation “": *expression"”. See **PEP 646**.

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 Lambdas.  Note that the lambda
expression is merely a shorthand for a simplified function definition;
a function defined in a “"def"” statement can be passed around or
assigned to another name just like a function defined by a lambda
expression.  The “"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 Naming and binding for details.

See also:

  **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**.
