Add TypeVar(infer_variance=True) (#80)

From PEP 695. Fixes #79.

Author: JelleZijlstra

CHANGELOG.md

@@ -1,9 +1,12 @@
 # Release 4.4.0 ()
+
 - Add `typing_extensions.Any` a backport of python 3.11's Any class which is
   subclassable at runtime. (backport from python/cpython#31841, by Shantanu 
   and Jelle Zijlstra). Patch by James Hilton-Balfe (@Gobot1234).
 - Add initial support for TypeVarLike `default` parameter, PEP 696.
   Patch by Marc Mueller (@cdce8p).
+- Add the `infer_variance` parameter to `TypeVar`, as specified in PEP 695.
+  Patch by Jelle Zijlstra.
 
 # Release 4.3.0 (July 1, 2022)
 

src/test_typing_extensions.py

@@ -3449,5 +3449,28 @@ def test_pickle(self):
                 self.assertEqual(z.__default__, typevar.__default__)
 
 
+class TypeVarInferVarianceTests(BaseTestCase):
+    def test_typevar(self):
+        T = typing_extensions.TypeVar('T')
+        self.assertFalse(T.__infer_variance__)
+        T_infer = typing_extensions.TypeVar('T_infer', infer_variance=True)
+        self.assertTrue(T_infer.__infer_variance__)
+        T_noinfer = typing_extensions.TypeVar('T_noinfer', infer_variance=False)
+        self.assertFalse(T_noinfer.__infer_variance__)
+
+    def test_pickle(self):
+        global U, U_infer  # pickle wants to reference the class by name
+        U = typing_extensions.TypeVar('U')
+        U_infer = typing_extensions.TypeVar('U_infer', infer_variance=True)
+        for proto in range(pickle.HIGHEST_PROTOCOL):
+            for typevar in (U, U_infer):
+                z = pickle.loads(pickle.dumps(typevar, proto))
+                self.assertEqual(z.__name__, typevar.__name__)
+                self.assertEqual(z.__covariant__, typevar.__covariant__)
+                self.assertEqual(z.__contravariant__, typevar.__contravariant__)
+                self.assertEqual(z.__bound__, typevar.__bound__)
+                self.assertEqual(z.__infer_variance__, typevar.__infer_variance__)
+
+
 if __name__ == '__main__':
     main()

src/typing_extensions.py

@@ -1163,18 +1163,19 @@ def __init__(self, default):
             self.__default__ = None
 
 
-# Add default Parameter - PEP 696
+# Add default and infer_variance parameters from PEP 696 and 695
 class TypeVar(typing.TypeVar, _DefaultMixin, _root=True):
     """Type variable."""
 
     __module__ = 'typing'
 
     def __init__(self, name, *constraints, bound=None,
                  covariant=False, contravariant=False,
-                 default=None):
+                 default=None, infer_variance=False):
         super().__init__(name, *constraints, bound=bound, covariant=covariant,
                          contravariant=contravariant)
         _DefaultMixin.__init__(self, default)
+        self.__infer_variance__ = infer_variance
 
         # for pickling:
         try: