Strict optional doesn't recognize "None in (...)" tests

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#2,980 6 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

The issue names no files or tests. Start by reproducing the snippet in mypy and trace how type narrowing handles membership tests; done means that None in (self.a, self.b) narrows both optionals enough for the addition to type-check, with regression coverage added.

Written by the indexing model from the issue text.

Description

feature priority-2-low topic-strict-optional topic-type-narrowing

A user reported code which basically boils down to this:

class A:
    a: Optional[int]
    b: Optional[int]
    ...
    def f(self) -> None:
        if None in (self.a, self.b):
            return
        print(self.a + self.b)  # ok at runtime but mypy error currently

Mypy doesn't recognize the None in (...) test as a legitimate None test.

The code seems a little unusual but if we see this frequently enough it might be worth supporting at some point.

Dominant language
Python
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