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, 2 insertions, 11 deletions

diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index b27165b..a398345 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -44,6 +44,7 @@ import shutil as _shutil
 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 @@ class SpooledTemporaryFile:
                                    '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