.. _tut-annotations:

Function Annotations

.. sectionauthor:: Zachary Ware <[email protected]>

.. index::

pair: function; annotations

single: -> (return annotation assignment)

:ref:`Function annotations <function>` are completely optional,

arbitrary metadata information about user-defined functions. Neither Python

itself nor the standard library use function annotations in any way; this

section just shows the syntax. Third-party projects are free to use function

annotations for documentation, type checking, and other uses.

Annotations are stored in the :attr:`__annotations__` attribute of the function

as a dictionary and have no effect on any other part of the function. Parameter

annotations are defined by a colon after the parameter name, followed by an

expression evaluating to the value of the annotation. Return annotations are

defined by a literal ``->``, followed by an expression, between the parameter

list and the colon denoting the end of the :keyword:`def` statement. The

following example has a positional argument, a keyword argument, and the return

value annotated with nonsense::