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`.

1 files changed, 3 insertions, 2 deletions

diff --git a/Lib/os.py b/Lib/os.py
index 8acd6f1..b794159 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -28,6 +28,8 @@ import stat as st
 
 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 @@ class PathLike(abc.ABC):
             return _check_methods(subclass, '__fspath__')
         return NotImplemented
 
-    def __class_getitem__(cls, type):
-        return cls
+    __class_getitem__ = classmethod(GenericAlias)
 
 
 if name == 'nt':