diff options
| -rw-r--r-- | Lib/test/test_typing.py | 11 | ||||
| -rw-r--r-- | Lib/typing.py | 15 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Library/2021-12-26-14-45-51.bpo-46170.AQ7kSM.rst | 1 |
3 files changed, 27 insertions, 0 deletions
diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py index 17d78cf..c76aa0a 100644 --- a/Lib/test/test_typing.py +++ b/Lib/test/test_typing.py @@ -4423,6 +4423,17 @@ class NewTypeTests: ) exec(code, {}) + def test_error_message_when_subclassing(self): + with self.assertRaisesRegex( + TypeError, + re.escape( + "Cannot subclass an instance of NewType. Perhaps you were looking for: " + "`ProUserId = NewType('ProUserId', UserId)`" + ) + ): + class ProUserId(UserId): + ... + class NewTypePythonTests(NewTypeTests, BaseTestCase): module = py_typing diff --git a/Lib/typing.py b/Lib/typing.py index 360129e..721afb7 100644 --- a/Lib/typing.py +++ b/Lib/typing.py @@ -2639,6 +2639,21 @@ class NewType: if def_mod != 'typing': self.__module__ = def_mod + def __mro_entries__(self, bases): + # We defined __mro_entries__ to get a better error message + # if a user attempts to subclass a NewType instance. bpo-46170 + superclass_name = self.__name__ + + class Dummy: + def __init_subclass__(cls): + subclass_name = cls.__name__ + raise TypeError( + f"Cannot subclass an instance of NewType. Perhaps you were looking for: " + f"`{subclass_name} = NewType({subclass_name!r}, {superclass_name})`" + ) + + return (Dummy,) + def __repr__(self): return f'{self.__module__}.{self.__qualname__}' diff --git a/Misc/NEWS.d/next/Library/2021-12-26-14-45-51.bpo-46170.AQ7kSM.rst b/Misc/NEWS.d/next/Library/2021-12-26-14-45-51.bpo-46170.AQ7kSM.rst new file mode 100644 index 0000000..5f266a2 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2021-12-26-14-45-51.bpo-46170.AQ7kSM.rst @@ -0,0 +1 @@ +Improve the error message when you try to subclass an instance of :class:`typing.NewType`. |