bpo-40398: Fix typing.get_args() for special generic aliases. (GH-19720)

Author: serhiy-storchaka

Lib/test/test_typing.py

@@ -3007,6 +3007,8 @@ class C(Generic[T]): pass
         self.assertIs(get_origin(List[Tuple[T, T]][int]), list)
         self.assertIs(get_origin(Annotated[T, 'thing']), Annotated)
         self.assertIs(get_origin(List), list)
+        self.assertIs(get_origin(Tuple), tuple)
+        self.assertIs(get_origin(Callable), collections.abc.Callable)
         self.assertIs(get_origin(list[int]), list)
         self.assertIs(get_origin(list), None)
 
@@ -3024,13 +3026,16 @@ class C(Generic[T]): pass
                          (int, Tuple[str, int]))
         self.assertEqual(get_args(typing.Dict[int, Tuple[T, T]][Optional[int]]),
                          (int, Tuple[Optional[int], Optional[int]]))
-        self.assertEqual(get_args(Callable[[], T][int]), ([], int,))
+        self.assertEqual(get_args(Callable[[], T][int]), ([], int))
+        self.assertEqual(get_args(Callable[..., int]), (..., int))
         self.assertEqual(get_args(Union[int, Callable[[Tuple[T, ...]], str]]),
                          (int, Callable[[Tuple[T, ...]], str]))
         self.assertEqual(get_args(Tuple[int, ...]), (int, ...))
         self.assertEqual(get_args(Tuple[()]), ((),))
         self.assertEqual(get_args(Annotated[T, 'one', 2, ['three']]), (T, 'one', 2, ['three']))
-        self.assertEqual(get_args(List), (typing.T,))
+        self.assertEqual(get_args(List), ())
+        self.assertEqual(get_args(Tuple), ())
+        self.assertEqual(get_args(Callable), ())
         self.assertEqual(get_args(list[int]), (int,))
         self.assertEqual(get_args(list), ())
 

Lib/typing.py

@@ -1420,11 +1420,13 @@ def get_args(tp):
     """
     if isinstance(tp, _AnnotatedAlias):
         return (tp.__origin__,) + tp.__metadata__
-    if isinstance(tp, (_GenericAlias, GenericAlias)):
+    if isinstance(tp, _GenericAlias) and not tp._special:
         res = tp.__args__
         if tp.__origin__ is collections.abc.Callable and res[0] is not Ellipsis:
             res = (list(res[:-1]), res[-1])
         return res
+    if isinstance(tp, GenericAlias):
+        return tp.__args__
     return ()
 
 

Misc/NEWS.d/next/Library/2020-04-26-22-25-36.bpo-40398.OdXnR3.rst

@@ -0,0 +1,2 @@
+:func:`typing.get_args` now always returns an empty tuple for special
+generic aliases.