bpo-44793: Fix checking the number of arguments when subscribe a generic type with ParamSpec parameter. (GH-27515)

For example Callable[P, T][[int], str, float] will now raise an error.

Use also term "arguments" instead of "parameters" in error
message for too few/many arguments.

Lib/test/test_typing.py

@@ -626,8 +626,6 @@ class TypingCallableTests(BaseCallableTests, BaseTestCase):
         self.assertEqual(c1.__args__, c2.__args__)
         self.assertEqual(hash(c1.__args__), hash(c2.__args__))
 
-    test_errors = skip("known bug #44793")(BaseCallableTests.test_errors)
-
 
 class CollectionsCallableTests(BaseCallableTests, BaseTestCase):
     Callable = collections.abc.Callable
@@ -4588,6 +4586,10 @@ class ParamSpecTests(BaseTestCase):
         G1 = X[int, P_2]
         self.assertEqual(G1.__args__, (int, P_2))
         self.assertEqual(G1.__parameters__, (P_2,))
+        with self.assertRaisesRegex(TypeError, "few arguments for"):
+            X[int]
+        with self.assertRaisesRegex(TypeError, "many arguments for"):
+            X[int, P_2, str]
 
         G2 = X[int, Concatenate[int, P_2]]
         self.assertEqual(G2.__args__, (int, Concatenate[int, P_2]))

Lib/typing.py

@@ -228,7 +228,7 @@ def _check_generic(cls, parameters, elen):
         raise TypeError(f"{cls} is not a generic class")
     alen = len(parameters)
     if alen != elen:
-        raise TypeError(f"Too {'many' if alen > elen else 'few'} parameters for {cls};"
+        raise TypeError(f"Too {'many' if alen > elen else 'few'} arguments for {cls};"
                         f" actual {alen}, expected {elen}")
 
 def _prepare_paramspec_params(cls, params):
@@ -239,6 +239,7 @@ def _prepare_paramspec_params(cls, params):
     if len(cls.__parameters__) == 1 and len(params) > 1:
         return (params,)
     else:
+        _check_generic(cls, params, len(cls.__parameters__))
         _params = []
         # Convert lists to tuples to help other libraries cache the results.
         for p, tvar in zip(params, cls.__parameters__):
@@ -1022,9 +1023,10 @@ class _GenericAlias(_BaseGenericAlias, _root=True):
         if not isinstance(params, tuple):
             params = (params,)
         params = tuple(_type_convert(p) for p in params)
-        if self._paramspec_tvars:
+        if (self._paramspec_tvars
-            if any(isinstance(t, ParamSpec) for t in self.__parameters__):
+                and any(isinstance(t, ParamSpec) for t in self.__parameters__)):
             params = _prepare_paramspec_params(self, params)
+        else:
             _check_generic(self, params, len(self.__parameters__))
 
         subst = dict(zip(self.__parameters__, params))
@@ -1292,6 +1294,7 @@ class Generic:
             # Subscripting a regular Generic subclass.
             if any(isinstance(t, ParamSpec) for t in cls.__parameters__):
                 params = _prepare_paramspec_params(cls, params)
+            else:
                 _check_generic(cls, params, len(cls.__parameters__))
         return _GenericAlias(cls, params,
                              _typevar_types=(TypeVar, ParamSpec),

Misc/NEWS.d/next/Library/2021-07-31-20-28-20.bpo-44793.woaQSg.rst

@@ -0,0 +1,2 @@
+Fix checking the number of arguments when subscribe a generic type with
+``ParamSpec`` parameter.