Add better default value for TypeVar default (PEP 696) (#85)

Author: cdce8p

CHANGELOG.md

@@ -1,3 +1,8 @@
+# Release 4.4.1 (?)
+
+- Add better default value for TypeVar `default` parameter, PEP 696. Enables
+  runtime check if `None` was passed as default. Patch by Marc Mueller (@cdce8p).
+
 # Release 4.4.0 (October 6, 2022)
 
 - Add `typing_extensions.Any` a backport of python 3.11's Any class which is

src/test_typing_extensions.py

@@ -3461,6 +3461,12 @@ def test_typevar(self):
         class A(Generic[T]): ...
         Alias = Optional[T]
 
+    def test_typevar_none(self):
+        U = typing_extensions.TypeVar('U')
+        U_None = typing_extensions.TypeVar('U_None', default=None)
+        self.assertEqual(U.__default__, None)
+        self.assertEqual(U_None.__default__, type(None))
+
     def test_paramspec(self):
         P = ParamSpec('P', default=(str, int))
         self.assertEqual(P.__default__, (str, int))

src/typing_extensions.py

@@ -1157,7 +1157,7 @@ def __init__(self, default):
         if isinstance(default, (tuple, list)):
             self.__default__ = tuple((typing._type_check(d, "Default must be a type")
                                       for d in default))
-        elif default:
+        elif default != _marker:
             self.__default__ = typing._type_check(default, "Default must be a type")
         else:
             self.__default__ = None
@@ -1171,7 +1171,7 @@ class TypeVar(typing.TypeVar, _DefaultMixin, _root=True):
 
     def __init__(self, name, *constraints, bound=None,
                  covariant=False, contravariant=False,
-                 default=None, infer_variance=False):
+                 default=_marker, infer_variance=False):
         super().__init__(name, *constraints, bound=bound, covariant=covariant,
                          contravariant=contravariant)
         _DefaultMixin.__init__(self, default)
@@ -1258,7 +1258,7 @@ class ParamSpec(typing.ParamSpec, _DefaultMixin, _root=True):
         __module__ = 'typing'
 
         def __init__(self, name, *, bound=None, covariant=False, contravariant=False,
-                     default=None):
+                     default=_marker):
             super().__init__(name, bound=bound, covariant=covariant,
                              contravariant=contravariant)
             _DefaultMixin.__init__(self, default)
@@ -1334,7 +1334,7 @@ def kwargs(self):
             return ParamSpecKwargs(self)
 
         def __init__(self, name, *, bound=None, covariant=False, contravariant=False,
-                     default=None):
+                     default=_marker):
             super().__init__([self])
             self.__name__ = name
             self.__covariant__ = bool(covariant)
@@ -1850,7 +1850,7 @@ def _is_unpack(obj):
     class TypeVarTuple(typing.TypeVarTuple, _DefaultMixin, _root=True):
         """Type variable tuple."""
 
-        def __init__(self, name, *, default=None):
+        def __init__(self, name, *, default=_marker):
             super().__init__(name)
             _DefaultMixin.__init__(self, default)
 
@@ -1913,7 +1913,7 @@ def get_shape(self) -> Tuple[*Ts]:
         def __iter__(self):
             yield self.__unpacked__
 
-        def __init__(self, name, *, default=None):
+        def __init__(self, name, *, default=_marker):
             self.__name__ = name
             _DefaultMixin.__init__(self, default)