Add a backport of generic NamedTuples

README.md

@@ -123,8 +123,7 @@ Certain objects were changed after they were added to `typing`, and
   `typing_extensions.get_overloads()`, you must use
   `@typing_extensions.overload`.
 - `NamedTuple` was changed in Python 3.11 to allow for multiple inheritance
-  with `typing.Generic`. The implementation of `NamedTuple` was also changed in
-  3.9 so that `NamedTuple` became a function rather than a class.
+  with `typing.Generic`.
 
 There are a few types whose interface was modified between different
 versions of typing. For example, `typing.Sequence` was modified to

src/test_typing_extensions.py

@@ -3114,12 +3114,7 @@ def test_namedtuple_errors(self):
     def test_copy_and_pickle(self):
         global Emp  # pickle wants to reference the class by name
         Emp = NamedTuple('Emp', [('name', str), ('cool', int)])
-        pickleable_classes = [Emp, CoolEmployee]
-        # support for pickling nested classes
-        # appears to have been added in 3.7.6
-        if sys.version_info >= (3, 7, 6):
-            pickleable_classes.append(self.NestedEmployee)
-        for cls in pickleable_classes:
+        for cls in Emp, CoolEmployee, self.NestedEmployee:
             with self.subTest(cls=cls):
                 jane = cls('jane', 37)
                 for proto in range(pickle.HIGHEST_PROTOCOL + 1):
@@ -3139,13 +3134,21 @@ def test_copy_and_pickle(self):
     def test_docstring(self):
         self.assertEqual(NamedTuple.__doc__, typing.NamedTuple.__doc__)
 
+    @skipIf(sys.version_info < (3, 8), "NamedTuple had a bad signature on <=3.7")
+    def test_signature_is_same_as_typing_NamedTuple(self):
+        self.assertEqual(inspect.signature(NamedTuple), inspect.signature(typing.NamedTuple))
+
+    @skipIf(sys.version_info >= (3, 8), "tests are only relveant to <=3.7")
+    def test_signature_on_37(self):
+        self.assertIsInstance(inspect.signature(NamedTuple), inspect.Signature)
+        self.assertFalse(hasattr(NamedTuple, "__text_signature__"))
+
     @skipIf(sys.version_info < (3, 9), "NamedTuple was a class on 3.8 and lower")
     def test_same_as_typing_NamedTuple_39_plus(self):
         self.assertEqual(
             set(dir(NamedTuple)),
             set(dir(typing.NamedTuple)) | {"__text_signature__"}
         )
-        self.assertEqual(inspect.signature(NamedTuple), inspect.signature(typing.NamedTuple))
         self.assertIs(type(NamedTuple), type(typing.NamedTuple))
 
     @skipIf(sys.version_info >= (3, 9), "tests are only relevant to <=3.8")
@@ -3154,8 +3157,6 @@ def test_same_as_typing_NamedTuple_38_minus(self):
             self.NestedEmployee.__annotations__,
             self.NestedEmployee._field_types
         )
-        self.assertIsInstance(inspect.signature(NamedTuple), inspect.Signature)
-        self.assertFalse(hasattr(NamedTuple, "__text_signature__"))
 
 
 if __name__ == '__main__':

src/typing_extensions.py

@@ -1961,6 +1961,9 @@ def decorator(cls_or_fn):
     typing._check_generic = _check_generic
 
 
+# Backport typing.NamedTuple as it exists in Python 3.11.
+# In 3.11, the ability to define generic `NamedTuple`s was supported.
+# This was explicitly disallowed in 3.9-3.10, and only half-worked in <=3.8.
 if sys.version_info >= (3, 11):
     NamedTuple = typing.NamedTuple
 else:
@@ -1984,7 +1987,7 @@ def _make_nmtuple(name, types, module, defaults=()):
         return nm_tpl
 
     _prohibited_namedtuple_fields = typing._prohibited
-    _special_namedtuple_fields = typing._special
+    _special_namedtuple_fields = frozenset({'__module__', '__name__', '__annotations__'})
 
     class _NamedTupleMeta(type):
         def __new__(cls, typename, bases, ns):
@@ -2034,7 +2037,10 @@ def NamedTuple(__typename, __fields=None, **kwargs):
     NamedTuple.__doc__ = typing.NamedTuple.__doc__
     _NamedTuple = type.__new__(_NamedTupleMeta, 'NamedTuple', (), {})
 
-    if sys.version_info >= (3, 9):
+    # On 3.8+, alter the signature so that it matches typing.NamedTuple.
+    # The signature of typing.NamedTuple on >=3.8 is invalid syntax in Python 3.7,
+    # so just leave the signature as it is on 3.7.
+    if sys.version_info >= (3, 8):
         NamedTuple.__text_signature__ = '(typename, fields=None, /, **kwargs)'
 
     def _namedtuple_mro_entries(bases):