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[3.9] bpo-42675: Document collections.abc.Callable changes (GH-23839) #23852

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gvanrossum merged 2 commits into from
Dec 19, 2020
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[3.9] bpo-42675: Document collections.abc.Callable changes (GH-23839) #23852

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c4bfd12
document for 3.9
Fidget-Spinner Dec 18, 2020
197ee2b
minor wording changes
Fidget-Spinner Dec 19, 2020
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3 changes: 3 additions & 0 deletions Doc/library/types.rst
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Expand Up @@ -260,6 +260,9 @@ Standard names are defined for the following types:

.. versionadded:: 3.9

.. versionchanged:: 3.9.2
This type can now be subclassed.


.. class:: TracebackType(tb_next, tb_frame, tb_lasti, tb_lineno)

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19 changes: 19 additions & 0 deletions Doc/whatsnew/3.9.rst
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Expand Up @@ -1497,3 +1497,22 @@ functions and options conditionally available based on the operating system
version in use at runtime ("weaklinking").

(Contributed by Ronald Oussoren and Lawrence D'Anna in :issue:`41100`.)

Notable changes in Python 3.9.2
===============================

collections.abc
---------------

:class:`collections.abc.Callable` generic now flattens type parameters, similar
to what :data:`typing.Callable` currently does. This means that
``collections.abc.Callable[[int, str], str]`` will have ``__args__`` of
``(int, str, str)``; previously this was ``([int, str], str)``. To allow this
change, :class:`types.GenericAlias` can now be subclassed, and a subclass will
be returned when subscripting the :class:`collections.abc.Callable` type.
Code which accesses the arguments via :func:`typing.get_args` or ``__args__``
need to account for this change. A :exc:`DeprecationWarning` may be emitted for
invalid forms of parameterizing :class:`collections.abc.Callable` which may have
passed silently in Python 3.9.1. This :exc:`DeprecationWarning` will
become a :exc:`TypeError` in Python 3.10.
(Contributed by Ken Jin in :issue:`42195`.)