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Enhance TypedDict docs around required/optional keys (#109547)

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As discussed in comments to #109544, the semantics of this attribute
are somewhat confusing. Add a note explaining its limitations and
steering users towards __required_keys__ and __optional_keys__ instead.
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Jelle Zijlstra 2023-09-27 10:35:46 -07:00 • committed by GitHub
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Doc/library/typing.rst
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@ -2404,6 +2404,13 @@ types.
>>> Point3D.__total__
True
This attribute reflects *only* the value of the ``total`` argument
to the current ``TypedDict`` class, not whether the class is semantically
total. For example, a ``TypedDict`` with ``__total__`` set to True may
have keys marked with :data:`NotRequired`, or it may inherit from another
``TypedDict`` with ``total=False``. Therefore, it is generally better to use
:attr:`__required_keys__` and :attr:`__optional_keys__` for introspection.
.. attribute:: __required_keys__
.. versionadded:: 3.9
@ -2439,6 +2446,14 @@ types.
.. versionadded:: 3.9
.. note::
If ``from __future__ import annotations`` is used or if annotations
are given as strings, annotations are not evaluated when the
``TypedDict`` is defined. Therefore, the runtime introspection that
``__required_keys__`` and ``__optional_keys__`` rely on may not work
properly, and the values of the attributes may be incorrect.
See :pep:`589` for more examples and detailed rules of using ``TypedDict``.
.. versionadded:: 3.8