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`.

Author: gvanrossum

Include/Python.h

@@ -120,6 +120,7 @@
 #include "iterobject.h"
 #include "genobject.h"
 #include "descrobject.h"
+#include "genericaliasobject.h"
 #include "warnings.h"
 #include "weakrefobject.h"
 #include "structseq.h"

Include/genericaliasobject.h

@@ -0,0 +1,14 @@
+// Implementation of PEP 585: support list[int] etc.
+#ifndef Py_GENERICALIASOBJECT_H
+#define Py_GENERICALIASOBJECT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+PyAPI_FUNC(PyObject *) Py_GenericAlias(PyObject *, PyObject *);
+PyAPI_DATA(PyTypeObject) Py_GenericAliasType;
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_GENERICALIASOBJECT_H */

Lib/_collections_abc.py

@@ -9,6 +9,8 @@
 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 @@ def __subclasshook__(cls, C):
             return _check_methods(C, "__await__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class Coroutine(Awaitable):
 
@@ -169,6 +173,8 @@ def __subclasshook__(cls, C):
             return _check_methods(C, "__aiter__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class AsyncIterator(AsyncIterable):
 
@@ -255,6 +261,8 @@ def __subclasshook__(cls, C):
             return _check_methods(C, "__iter__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class Iterator(Iterable):
 
@@ -274,6 +282,7 @@ def __subclasshook__(cls, C):
             return _check_methods(C, '__iter__', '__next__')
         return NotImplemented
 
+
 Iterator.register(bytes_iterator)
 Iterator.register(bytearray_iterator)
 #Iterator.register(callable_iterator)
@@ -353,6 +362,7 @@ def __subclasshook__(cls, C):
                                   'send', 'throw', 'close')
         return NotImplemented
 
+
 Generator.register(generator)
 
 
@@ -385,6 +395,9 @@ def __subclasshook__(cls, C):
             return _check_methods(C, "__contains__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
+
 class Collection(Sized, Iterable, Container):
 
     __slots__ = ()
@@ -395,6 +408,7 @@ def __subclasshook__(cls, C):
             return _check_methods(C,  "__len__", "__iter__", "__contains__")
         return NotImplemented
 
+
 class Callable(metaclass=ABCMeta):
 
     __slots__ = ()
@@ -409,6 +423,8 @@ def __subclasshook__(cls, C):
             return _check_methods(C, "__call__")
         return NotImplemented
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 ### SETS ###
 
@@ -550,6 +566,7 @@ def _hash(self):
             h = 590923713
         return h
 
+
 Set.register(frozenset)
 
 
@@ -632,6 +649,7 @@ def __isub__(self, it):
                 self.discard(value)
         return self
 
+
 MutableSet.register(set)
 
 
@@ -688,6 +706,7 @@ def __eq__(self, other):
 
     __reversed__ = None
 
+
 Mapping.register(mappingproxy)
 
 
@@ -704,6 +723,8 @@ def __len__(self):
     def __repr__(self):
         return '{0.__class__.__name__}({0._mapping!r})'.format(self)
 
+    __class_getitem__ = classmethod(GenericAlias)
+
 
 class KeysView(MappingView, Set):
 
@@ -719,6 +740,7 @@ def __contains__(self, key):
     def __iter__(self):
         yield from self._mapping
 
+
 KeysView.register(dict_keys)
 
 
@@ -743,6 +765,7 @@ def __iter__(self):
         for key in self._mapping:
             yield (key, self._mapping[key])
 
+
 ItemsView.register(dict_items)
 
 
@@ -761,6 +784,7 @@ def __iter__(self):
         for key in self._mapping:
             yield self._mapping[key]
 
+
 ValuesView.register(dict_values)
 
 
@@ -847,6 +871,7 @@ def setdefault(self, key, default=None):
             self[key] = default
         return default
 
+
 MutableMapping.register(dict)
 
 
@@ -914,6 +939,7 @@ def count(self, value):
         '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 @@ def __iadd__(self, values):
         self.extend(values)
         return self
 
+
 MutableSequence.register(list)
 MutableSequence.register(bytearray)  # Multiply inheriting, see ByteString

Lib/contextlib.py

@@ -4,7 +4,7 @@
 import _collections_abc
 from collections import deque
 from functools import wraps
-from types import MethodType
+from types import MethodType, GenericAlias
 
 __all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
            "AbstractContextManager", "AbstractAsyncContextManager",
@@ -16,6 +16,8 @@ class AbstractContextManager(abc.ABC):
 
     """An abstract base class for context managers."""
 
+    __class_getitem__ = classmethod(GenericAlias)
+
     def __enter__(self):
         """Return `self` upon entering the runtime context."""
         return self
@@ -36,6 +38,8 @@ class AbstractAsyncContextManager(abc.ABC):
 
     """An abstract base class for asynchronous context managers."""
 
+    __class_getitem__ = classmethod(GenericAlias)
+
     async def __aenter__(self):
         """Return `self` upon entering the runtime context."""
         return self

Lib/os.py

@@ -28,6 +28,8 @@
 
 from _collections_abc import _check_methods
 
+GenericAlias = type(list[int])
+
 _names = sys.builtin_module_names
 
 # Note:  more names are added to __all__ later.
@@ -1074,8 +1076,7 @@ def __subclasshook__(cls, subclass):
             return _check_methods(subclass, '__fspath__')
         return NotImplemented
 
-    def __class_getitem__(cls, type):
-        return cls
+    __class_getitem__ = classmethod(GenericAlias)
 
 
 if name == 'nt':

Lib/subprocess.py

@@ -52,6 +52,7 @@
 import warnings
 import contextlib
 from time import monotonic as _time
+import types
 
 try:
     import pwd
@@ -446,17 +447,7 @@ def __repr__(self):
             args.append('stderr={!r}'.format(self.stderr))
         return "{}({})".format(type(self).__name__, ', '.join(args))
 
-    def __class_getitem__(cls, type):
-        """Provide minimal support for using this class as generic
-        (for example in type annotations).
-
-        See PEP 484 and PEP 560 for more details. For example,
-        `CompletedProcess[bytes]` is a valid expression at runtime
-        (type argument `bytes` indicates the type used for stdout).
-        Note, no type checking happens at runtime, but a static type
-        checker can be used.
-        """
-        return cls
+    __class_getitem__ = classmethod(types.GenericAlias)
 
 
     def check_returncode(self):
@@ -1000,16 +991,7 @@ def __repr__(self):
             obj_repr = obj_repr[:76] + "...>"
         return obj_repr
 
-    def __class_getitem__(cls, type):
-        """Provide minimal support for using this class as generic
-        (for example in type annotations).
-
-        See PEP 484 and PEP 560 for more details. For example, `Popen[bytes]`
-        is a valid expression at runtime (type argument `bytes` indicates the
-        type used for stdout). Note, no type checking happens at runtime, but
-        a static type checker can be used.
-        """
-        return cls
+    __class_getitem__ = classmethod(types.GenericAlias)
 
     @property
     def universal_newlines(self):

Lib/tempfile.py

@@ -44,6 +44,7 @@
 import errno as _errno
 from random import Random as _Random
 import sys as _sys
+import types as _types
 import weakref as _weakref
 import _thread
 _allocate_lock = _thread.allocate_lock
@@ -643,17 +644,7 @@ def __init__(self, max_size=0, mode='w+b', buffering=-1,
                                    'encoding': encoding, 'newline': newline,
                                    'dir': dir, 'errors': errors}
 
-    def __class_getitem__(cls, type):
-        """Provide minimal support for using this class as generic
-        (for example in type annotations).
-
-        See PEP 484 and PEP 560 for more details. For example,
-        `SpooledTemporaryFile[str]` is a valid expression at runtime (type
-        argument `str` indicates whether the file is open in bytes or text
-        mode). Note, no type checking happens at runtime, but a static type
-        checker can be used.
-        """
-        return cls
+    __class_getitem__ = classmethod(_types.GenericAlias)
 
     def _check(self, file):
         if self._rolled: return

Lib/test/test_descrtut.py

@@ -167,6 +167,7 @@ def merge(self, other):
     >>> pprint.pprint(dir(list))    # like list.__dict__.keys(), but sorted
     ['__add__',
      '__class__',
+     '__class_getitem__',
      '__contains__',
      '__delattr__',
      '__delitem__',

Lib/test/test_doctest.py

@@ -665,7 +665,7 @@ def non_Python_modules(): r"""
 
     >>> import builtins
     >>> tests = doctest.DocTestFinder().find(builtins)
-    >>> 800 < len(tests) < 820 # approximate number of objects with docstrings
+    >>> 810 < len(tests) < 830 # approximate number of objects with docstrings
     True
     >>> real_tests = [t for t in tests if len(t.examples) > 0]
     >>> len(real_tests) # objects that actually have doctests