[Python-checkins] bpo-40397: Fix subscription of nested generic alias without parameters. (GH-20021)

[Serhiy Storchaka]

https://github.com/python/cpython/commit/0122d48681b1df27015cf396559fb283ba364d6d
commit: 0122d48681b1df27015cf396559fb283ba364d6d
branch: master
author: Serhiy Storchaka <storchaka at gmail.com>
committer: GitHub <noreply at github.com>
date: 2020-05-10T13:39:40+03:00
summary:

bpo-40397: Fix subscription of nested generic alias without parameters. (GH-20021)

files:
M Lib/test/test_typing.py
M Lib/typing.py

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index 21bc7c81f2a30..f429e883b5953 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -10,7 +10,7 @@
 from typing import TypeVar, AnyStr
 from typing import T, KT, VT  # Not in __all__.
 from typing import Union, Optional, Literal
-from typing import Tuple, List, MutableMapping
+from typing import Tuple, List, Dict, MutableMapping
 from typing import Callable
 from typing import Generic, ClassVar, Final, final, Protocol
 from typing import cast, runtime_checkable
@@ -3173,6 +3173,17 @@ def test_frozenset(self):
     def test_dict(self):
         self.assertIsSubclass(dict, typing.Dict)
 
+    def test_dict_subscribe(self):
+        K = TypeVar('K')
+        V = TypeVar('V')
+        self.assertEqual(Dict[K, V][str, int], Dict[str, int])
+        self.assertEqual(Dict[K, int][str], Dict[str, int])
+        self.assertEqual(Dict[str, V][int], Dict[str, int])
+        self.assertEqual(Dict[K, List[V]][str, int], Dict[str, List[int]])
+        self.assertEqual(Dict[K, List[int]][str], Dict[str, List[int]])
+        self.assertEqual(Dict[K, list[V]][str, int], Dict[str, list[int]])
+        self.assertEqual(Dict[K, list[int]][str], Dict[str, list[int]])
+
     def test_no_list_instantiation(self):
         with self.assertRaises(TypeError):
             typing.List()
diff --git a/Lib/typing.py b/Lib/typing.py
index e31fc99e02245..b5ba38e07c835 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -702,8 +702,10 @@ def __getitem__(self, params):
             if isinstance(arg, TypeVar):
                 arg = subst[arg]
             elif isinstance(arg, (_GenericAlias, GenericAlias)):
-                subargs = tuple(subst[x] for x in arg.__parameters__)
-                arg = arg[subargs]
+                subparams = arg.__parameters__
+                if subparams:
+                    subargs = tuple(subst[x] for x in subparams)
+                    arg = arg[subargs]
             new_args.append(arg)
         return self.copy_with(tuple(new_args))