bpo-39481: Implementation for PEP 585 (#18239)

This implements things like `list[int]`,
which returns an object of type `types.GenericAlias`.
This object mostly acts as a proxy for `list`,
but has attributes `__origin__` and `__args__`
that allow recovering the parts (with values `list` and `(int,)`.

There is also an approximate notion of type variables;
e.g. `list[T]` has a `__parameters__` attribute equal to `(T,)`.
Type variables are objects of type `typing.TypeVar`.

Lib/_collections_abc.py

@@ -9,6 +9,8 @@ Unit tests are in test_collections.
 from abc import ABCMeta, abstractmethod
 import sys
 
+GenericAlias = type(list[int])
+
 __all__ = ["Awaitable", "Coroutine",
            "AsyncIterable", "AsyncIterator", "AsyncGenerator",
            "Hashable", "Iterable", "Iterator", "Generator", "Reversible",
@@ -110,6 +112,8 @@ class Awaitable(metaclass=ABCMeta):
             return _check_methods(C, "__await__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class Coroutine(Awaitable):
 
@@ -169,6 +173,8 @@ class AsyncIterable(metaclass=ABCMeta):
             return _check_methods(C, "__aiter__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class AsyncIterator(AsyncIterable):
 
@@ -255,6 +261,8 @@ class Iterable(metaclass=ABCMeta):
             return _check_methods(C, "__iter__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class Iterator(Iterable):
 
@@ -274,6 +282,7 @@ class Iterator(Iterable):
             return _check_methods(C, '__iter__', '__next__')
         return NotImplemented
 
+
 Iterator.register(bytes_iterator)
 Iterator.register(bytearray_iterator)
 #Iterator.register(callable_iterator)
@@ -353,6 +362,7 @@ class Generator(Iterator):
                                   'send', 'throw', 'close')
         return NotImplemented
 
+
 Generator.register(generator)
 
 
@@ -385,6 +395,9 @@ class Container(metaclass=ABCMeta):
             return _check_methods(C, "__contains__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
+
 class Collection(Sized, Iterable, Container):
 
     __slots__ = ()
@@ -395,6 +408,7 @@ class Collection(Sized, Iterable, Container):
             return _check_methods(C,  "__len__", "__iter__", "__contains__")
         return NotImplemented
 
+
 class Callable(metaclass=ABCMeta):
 
     __slots__ = ()
@@ -409,6 +423,8 @@ class Callable(metaclass=ABCMeta):
             return _check_methods(C, "__call__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 ### SETS ###
 
@@ -550,6 +566,7 @@ class Set(Collection):
             h = 590923713
         return h
 
+
 Set.register(frozenset)
 
 
@@ -632,6 +649,7 @@ class MutableSet(Set):
                 self.discard(value)
         return self
 
+
 MutableSet.register(set)
 
 
@@ -688,6 +706,7 @@ class Mapping(Collection):
 
     __reversed__ = None
 
+
 Mapping.register(mappingproxy)
 
 
@@ -704,6 +723,8 @@ class MappingView(Sized):
     def __repr__(self):
         return '{0.__class__.__name__}({0._mapping!r})'.format(self)
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class KeysView(MappingView, Set):
 
@@ -719,6 +740,7 @@ class KeysView(MappingView, Set):
     def __iter__(self):
         yield from self._mapping
 
+
 KeysView.register(dict_keys)
 
 
@@ -743,6 +765,7 @@ class ItemsView(MappingView, Set):
         for key in self._mapping:
             yield (key, self._mapping[key])
 
+
 ItemsView.register(dict_items)
 
 
@@ -761,6 +784,7 @@ class ValuesView(MappingView, Collection):
         for key in self._mapping:
             yield self._mapping[key]
 
+
 ValuesView.register(dict_values)
 
 
@@ -847,6 +871,7 @@ class MutableMapping(Mapping):
             self[key] = default
         return default
 
+
 MutableMapping.register(dict)
 
 
@@ -914,6 +939,7 @@ class Sequence(Reversible, Collection):
         'S.count(value) -> integer -- return number of occurrences of value'
         return sum(1 for v in self if v is value or v == value)
 
+
 Sequence.register(tuple)
 Sequence.register(str)
 Sequence.register(range)
@@ -1000,5 +1026,6 @@ class MutableSequence(Sequence):
         self.extend(values)
         return self
 
+
 MutableSequence.register(list)
 MutableSequence.register(bytearray)  # Multiply inheriting, see ByteString