bpo-32717: Document PEP 560 (GH-6726)

Doc/reference/datamodel.rst

@@ -1857,11 +1857,27 @@ passed through to all metaclass operations described below.
 
 When a class definition is executed, the following steps occur:
 
+* MRO entries are resolved
 * the appropriate metaclass is determined
 * the class namespace is prepared
 * the class body is executed
 * the class object is created
 
+
+Resolving MRO entries
+^^^^^^^^^^^^^^^^^^^^^
+
+If a base that appears in class definition is not an instance of :class:`type`,
+then an ``__mro_entries__`` method is searched on it. If found, it is called
+with the original bases tuple. This method must return a tuple of classes that
+will be used instead of this base. The tuple may be empty, in such case
+the original base is ignored.
+
+.. seealso::
+
+   :pep:`560` - Core support for typing module and generic types
+
+
 Determining the appropriate metaclass
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 .. index::
@@ -2061,6 +2077,27 @@ case the instance is itself a class.
       module) to the language.
 
 
+Emulating generic types
+-----------------------
+
+One can implement the generic class syntax as specified by :pep:`484`
+(for example ``List[int]``) by defining a special method
+
+.. 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.
+
+.. seealso::
+
+   :pep:`560` - Core support for typing module and generic types
+
+
 .. _callable-types:
 
 Emulating callable objects