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Better error message when union may not have been narrowed #17036
Description
Feature
If some attribute is not defined for a single union item (often None), we could give a hint that the type may need to be narrowed down.
Pitch
New users don't always figure out that they need an isinstance or an is not None check, and mypy could make this easier for beginners.
Toy example where this could apply:
def f(s: str | None) -> bool:
# Item "None" of "str | None" has no attribute "startswith"
return s.startswith('x')I'm not sure what a good hint could be, but here's an initial idea:
x.py:3: note: You can use "if s is not None" to guard against a None value
Activity
Eben-Success commentedon Mar 22, 2024
I want to fix this issue.
keshavt3 commentedon Apr 3, 2024
I want to work on this.
kr321 commentedon Apr 8, 2024
I would like to work on this with a group for a college software engineering class.
hauntsaninja commentedon Apr 8, 2024
Any of you please feel free to, just post on this issue when you have an open PR that passes tests!
GabrielAguirre1 commentedon Apr 9, 2024
I would similarly like to work on it.
biswaisop commentedon Jun 18, 2024
I want to fix this
Prabhat-Thapa45 commentedon Jul 24, 2024
I felt many people missed what you( @JukkaL ) meant by often None to only none. I have added hint for all the missing attribute in my pr.
Issue python#17036 fixed
paulorochaoliveira commentedon Dec 6, 2024
I believe this issue can be closed.
brianschubert commentedon Dec 6, 2024
I don't think a PR for this issue has been merged yet (the merged PR above is on a fork).
paulorochaoliveira commentedon Dec 6, 2024
@brianschubert You're right. My bad.
Suggest narrowing for None in union attribute errors
hauntsaninja commentedon Mar 27, 2026
I think mypy's behaviour is fine as-is. It's not clear that this is an issue users are confused about, or that recommending an
ifis better than recommending anassert