gh-127537: Add __class_getitem__ to the python implementation of functools.partial (#127537)

Author: cfbolz

Lib/functools.py

@@ -433,6 +433,9 @@ def __setstate__(self, state):
         self._phcount = phcount
         self._merger = merger
 
+    __class_getitem__ = classmethod(GenericAlias)
+
+
 try:
     from _functools import partial, Placeholder, _PlaceholderType
 except ImportError:

Lib/test/test_functools.py

@@ -473,6 +473,12 @@ class A:
         self.assertEqual(a.cmeth(3, b=4), ((1, A, 3), {'a': 2, 'b': 4}))
         self.assertEqual(a.smeth(3, b=4), ((1, 3), {'a': 2, 'b': 4}))
 
+    def test_partial_genericalias(self):
+        alias = self.partial[int]
+        self.assertIs(alias.__origin__, self.partial)
+        self.assertEqual(alias.__args__, (int,))
+        self.assertEqual(alias.__parameters__, ())
+
 
 @unittest.skipUnless(c_functools, 'requires the C _functools module')
 class TestPartialC(TestPartial, unittest.TestCase):

Misc/NEWS.d/next/Library/2024-12-04-10-39-29.gh-issue-83662.CG1s3m.rst

@@ -0,0 +1,5 @@
+Add missing ``__class_getitem__`` method to the Python implementation of
+:func:`functools.partial`, to make it compatible with the C version. This is
+mainly relevant for alternative Python implementations like PyPy and
+GraalPy, because CPython will usually use the C-implementation of that
+function.