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[3.13] gh-90190: Add doc for using singledispatch with precise collection type hints (GH-116544) #124710

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Yhg1s merged 1 commit into from
Sep 27, 2024
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[3.13] gh-90190: Add doc for using singledispatch with precise collection type hints (GH-116544) #124710

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gh-90190: Add doc for using singledispatch with precise collection … 
…type hints (GH-116544)

(cherry picked from commit 2357d5b)

Co-authored-by: Matt Delengowski <matt.delengowski@gmail.com>
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@Delengowski @miss-islington
Delengowski authored and miss-islington committed Sep 27, 2024
commit 0864743a4289abe8d46d41d288c56ba2ffb4df00
19 changes: 19 additions & 0 deletions Doc/library/functools.rst
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Expand Up @@ -492,6 +492,25 @@ The :mod:`functools` module defines the following functions:
... print(arg.real, arg.imag)
...

For code that dispatches on a collections type (e.g., ``list``), but wants
to typehint the items of the collection (e.g., ``list[int]``), the
dispatch type should be passed explicitly to the decorator itself with the
typehint going into the function definition::

>>> @fun.register(list)
... def _(arg: list[int], verbose=False):
... if verbose:
... print("Enumerate this:")
... for i, elem in enumerate(arg):
... print(i, elem)

.. note::

At runtime the function will dispatch on an instance of a list regardless
of the type contained within the list i.e. ``[1,2,3]`` will be
dispatched the same as ``["foo", "bar", "baz"]``. The annotation
provided in this example is for static type checkers only and has no
runtime impact.

To enable registering :term:`lambdas<lambda>` and pre-existing functions,
the :func:`register` attribute can also be used in a functional form::
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