[doc] bpo-45680: Disambiguate __getitem__ and ``__class_getitem__…

…`` in the data model. The documentation explaining Python's data model does not adequately explain the differences between ``__getitem__`` and ``__class_getitem__``, nor does it explain when each is called. There is an attempt at explaining ``__class_getitem__`` in the documentation for ``GenericAlias`` objects, but this does not give sufficient clarity into how the method works. Moreover, it is the wrong place for that information to be found; the explanation of ``__class_getitem__`` should be in the documentation explaining the data model. This PR has been split off from #29335.
python · ambv · Nov 18, 2021 · Nov 3, 2021 · Nov 3, 2021 · Nov 3, 2021
commit 020d1681ec4dd43b73d228f401015011a7c4a0cc
@@ -2215,23 +2215,80 @@ case the instance is itself a class.
 Emulating generic types
 -----------------------
 
-One can implement the generic class syntax as specified by :pep:`484`
-(for example ``List[int]``) by defining a special method:
+When using :term:`type annotations<annotation>`, it is often useful to
+*parameterize* a generic type using Python's square-brackets notation.
+For example, the annotation ``list[int]`` might be used to signify a
+:class:`list` in which all the elements are of type :class:`int`.
+
+.. seealso::
+
+   :pep:`484` - Type Hints
+      Introducing Python's framework for type annotations
+
+   :ref:`Generic Alias Types<types-genericalias>`
+      Documentation for objects representing parameterized generic classes
+
+   :class:`typing.Generic`
+      Inherit from :class:`typing.Generic` to implement generic classes that
+      can be parameterized at runtime and understood by static type-checkers.
+
+A class can generally only be parameterized if it defines the special
+classmethod ``__class_getitem__()``.
 
 .. classmethod:: object.__class_getitem__(cls, key)
 
    Return an object representing the specialization of a generic class
    by type arguments found in *key*.
 
-This method is looked up on the class object itself, and when defined in
-the class body, this method is implicitly a class method.  Note, this
-mechanism is primarily reserved for use with static type hints, other usage
-is discouraged.
+``__getitem__`` *versus* ``__class_getitem__``
 
-.. seealso::
+   Usually, the :ref:`subscription <subscriptions>` of an object in Python
+   using the square-brackets notation will call the :meth:`~object.__getitem__`
+   instance method defined on the object's class.
 
-   :pep:`560` - Core support for typing module and generic types
+   For example, if we have a list ``food`` as follows::
+
+      food = ['spam', 'eggs', 'bacon']
+
+   Calling ``food[0]`` will return the same value as calling::
+
+      type(food).__getitem__(food, 0)
+
+   However, if a class defines the classmethod ``__class_getitem__()``, then
+   the subscription of that class may call the class's implementation of
+   ``__class_getitem__()`` rather than :meth:`~object.__getitem__`.
+   ``__class_getitem__()`` should return a
+   :ref:`GenericAlias<types-genericalias>` object if it is properly defined.
 
+   For example, because the :class:`list` class defines
+   ``__class_getitem__()``, calling ``list[str]`` is equivalent to calling::
+
+      list.__class_getitem__(str)
+
+   rather than::
+
+      type(list).__getitem__(list, str)
+
+.. note::
+   If :meth:`~object.__getitem__` is defined by a class's :term:`metaclass`, it
+   will take precedence over a ``__class_getitem__()`` classmethod
+   defined by the class. See :pep:`560` for more details.
+
+.. note::
+   ``__class_getitem__()`` was introduced to implement runtime parameterization
+   of standard-library generic classes in order to more easily apply
+   :term:`type-hints<type hint>` to these classes.
+
+   To implement custom generic classes that can be parameterized at runtime and
+   understood by static type-checkers, users should either inherit from a
+   standard library class that already implements ``__class_getitem__()``, or
+   inherit from :class:`typing.Generic`, which has its own implementation of
+   ``__class_getitem__()``.
+
+   Custom implementations of ``__class_getitem__()`` on classes defined outside
+   of the standard library may not be understood by third-party type-checkers
+   such as mypy. Using ``__class_getitem__()`` on any class for purposes other
+   than type-hinting is discouraged.
 
 .. _callable-types:
 
@@ -2344,6 +2401,12 @@ through the object's keys; for sequences, it should iterate through the values.
       :keyword:`for` loops expect that an :exc:`IndexError` will be raised for illegal
       indexes to allow proper detection of the end of the sequence.
 
+   .. note::
+
+      When :ref:`subscripting<subscriptions>` a *class*, the special
+      classmethod :meth:`~object.__class_getitem__` may be called instead of
+      ``__getitem__()``.
+
 
 .. method:: object.__setitem__(self, key, value)
 

diff --git a/Misc/NEWS.d/next/Documentation/2021-11-03-14-30-55.bpo-45680.UBpWzY.rst b/Misc/NEWS.d/next/Documentation/2021-11-03-14-30-55.bpo-45680.UBpWzY.rst
@@ -0,0 +1,2 @@
+Disambiguate the differences between ``__getitem__`` and __class_getitem__``
+in the documentation for the data model. Patch by Alex Waygood.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Disambiguate the differences between ``__getitem__`` and __class_getitem__``
		in the documentation for the data model. Patch by Alex Waygood.