Don't turn list defaults for ParamSpecs into tuples (#394)

Author: AlexWaygood

CHANGELOG.md

@@ -18,6 +18,9 @@
 - It is now disallowed to use a `TypeVar` with a default value after a
   `TypeVarTuple` in a type parameter list. This matches the CPython
   implementation of PEP 696 on Python 3.13+.
+- Fix bug in PEP-696 implementation where default values for `ParamSpec`s
+  would be cast to tuples if a list was provided as the default value.
+  Patch by Alex Waygood.
 - Fix `Protocol` tests on Python 3.13.0a6 and newer. 3.13.0a6 adds a new
   `__static_attributes__` attribute to all classes in Python,
   which broke some assumptions made by the implementation of

src/test_typing_extensions.py

@@ -4997,7 +4997,7 @@ def test_pickle(self):
         P = ParamSpec('P')
         P_co = ParamSpec('P_co', covariant=True)
         P_contra = ParamSpec('P_contra', contravariant=True)
-        P_default = ParamSpec('P_default', default=int)
+        P_default = ParamSpec('P_default', default=[int])
         for proto in range(pickle.HIGHEST_PROTOCOL):
             with self.subTest(f'Pickle protocol {proto}'):
                 for paramspec in (P, P_co, P_contra, P_default):
@@ -6363,8 +6363,8 @@ def test_typevar_none(self):
         self.assertTrue(U_None.has_default())
 
     def test_paramspec(self):
-        P = ParamSpec('P', default=(str, int))
-        self.assertEqual(P.__default__, (str, int))
+        P = ParamSpec('P', default=[str, int])
+        self.assertEqual(P.__default__, [str, int])
         self.assertTrue(P.has_default())
         self.assertIsInstance(P, ParamSpec)
         if hasattr(typing, "ParamSpec"):
@@ -6457,6 +6457,17 @@ def test_pickle(self):
                 self.assertEqual(z.__bound__, typevar.__bound__)
                 self.assertEqual(z.__default__, typevar.__default__)
 
+    def test_strange_defaults_are_allowed(self):
+        # Leave it to type checkers to check whether strange default values
+        # should be allowed or disallowed
+        def not_a_type(): ...
+
+        for typevarlike_cls in TypeVar, ParamSpec, TypeVarTuple:
+            for default in not_a_type, 42, bytearray(), (int, not_a_type, 42):
+                with self.subTest(typevarlike_cls=typevarlike_cls, default=default):
+                    T = typevarlike_cls("T", default=default)
+                    self.assertEqual(T.__default__, default)
+
     @skip_if_py313_beta_1
     def test_allow_default_after_non_default_in_alias(self):
         T_default = TypeVar('T_default', default=int)

src/typing_extensions.py

@@ -1467,16 +1467,7 @@ def __reduce__(self):
 
 def _set_default(type_param, default):
     type_param.has_default = lambda: default is not NoDefault
-    if isinstance(default, (tuple, list)):
-        type_param.__default__ = tuple((typing._type_check(d, "Default must be a type")
-                                        for d in default))
-    elif default in (None, NoDefault):
-        type_param.__default__ = default
-    else:
-        if isinstance(type_param, ParamSpec) and default is ...:  # ... not valid <3.11
-            type_param.__default__ = default
-        else:
-            type_param.__default__ = typing._type_check(default, "Default must be a type")
+    type_param.__default__ = default
 
 
 def _set_module(typevarlike):