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unittest style asserts don't have same type narrowing behaviour as normal asserts #2181
Closed as duplicate
Description
Summary
unittest-style self.assert... is not narrowing types as I'd expect.
I have just seen this diagnostic:
error[non-subscriptable]: Cannot subscript object of type `None` with no `__getitem__` method
--> tests\test_builder.py:304:27
|
302 | # Assert
303 | self.assertIsNotNone(publisher.message)
304 | self.assertIsNone(publisher.message["ElapsedTimeMs"])
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
info: rule `non-subscriptable` is enabled by default
I would expect the self.assertIsNotNone to make line 304 valid. If I change the first like to assert publisher.message is not None then no error is flagged.
target python version: 3.12
ty version: ty 0.0.5 (d37b7db 2025-12-20)
Thanks!
Version
ty 0.0.5 (d37b7db 2025-12-20)
Activity
AlexWaygood commentedon Dec 23, 2025
Thanks for the report!
You haven't provided us with a minimal repro here, but I'm guessing your code looks something like this?
If so, this is not a form of narrowing supported by any other type checker right now. I think the real solution to this is python/typing#930, which would allow typeshed to annotate the return type of methods like
assertIsNotNonein such a way that all type checkers would understand their narrowing behaviour.joe-pierce commentedon Dec 23, 2025
Yes that code snippet reflects my raised issue - sounds like a resolution to this sits outside this project so feel free to close, unless you think there may be some improvements to the returned diagnostic in the terminal.