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bpo-34498: Warn against using deprecated typing aliases with @singledispatch #29508

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bpo-34498: Warn against using deprecated typing aliases with @singledispatch #29508

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bpo-34498: Warn against using deprecated ``typing`` aliases with ``@s…
AlexWaygood Nov 9, 2021
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AlexWaygood Dec 4, 2021
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11 changes: 11 additions & 0 deletions Doc/library/functools.rst
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Expand Up @@ -430,6 +430,17 @@ The :mod:`functools` module defines the following functions:
... for i, elem in enumerate(arg):
... print(i, elem)

.. note::
The :mod:`typing` module provides a number of aliases to
builtin classes, as well as classes found elsewhere in the standard
library. All have been deprecated since Python 3.9 (see :pep:`585`).
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AlexWaygood marked this conversation as resolved.
Annotating the first argument of a ``@singledispatch`` implementation
with one of these aliases is not supported, and will lead to a
:exc:`TypeError` being raised in Python 3.7 and above. Use
:pep:`585`-style syntax instead: ``list`` rather than ``typing.List``,
``collections.abc.Iterable[str]`` rather than ``typing.Iterable[str]``,
etc.

For code which doesn't use type annotations, the appropriate type
argument can be passed explicitly to the decorator itself::

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Documentation/2021-11-09-22-20-54.bpo-34498.oTiZ46.rst
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@@ -0,0 +1,3 @@
Add note to :func:`functools.singledispatch` documentation warning that
attempting to register an implementation with a deprecated alias from the
:mod:`typing` module is not supported.