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Fix crash with function redefinition #14064

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merged 2 commits into from Nov 11, 2022

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hauntsaninja
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@hauntsaninja hauntsaninja commented Nov 11, 2022

Fixes #14027 (issue was surfaced by #13509)

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@JelleZijlstra JelleZijlstra left a comment

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Seems OK as quick fix but the error message is still pretty bad.

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@ilevkivskyi ilevkivskyi left a comment

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Don't want to block this, but I think I have a better solution, see comment below.

# weirdly, needs to be an empty tuple. empty lists and various other special assignments
# don't trigger the bug
cond = ()
if cond:
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A simpler test would be just if False: The problem is that mypy considers if branch unreachable, and therefore doesn't type check it, and therefore the original definition doesn't get its type inferred (and stays None).

I think a better solution is to simply not show any error in this case:

  • It will be more consistent with not showing error in unreachable code
  • Technically, there is no redefinition, only one definition is ever executed.

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Note however, this will be inconsistent with foo: int = 1, where user will get an error, even if the first definition is unreachable, but IMO this is OK.

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Btw it is a wider problem, some code may be semantically reachable (because types of expressions in branches are not known), but not type checked, because during type-checking expression type is inferred as something false-y. A possible option to explore in the future (not now) is to still actually type check unreachable code, only for the purpose of type inference, but don't show any errors there.

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According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

@hauntsaninja hauntsaninja merged commit 67855fa into python:master Nov 11, 2022
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@hauntsaninja hauntsaninja deleted the fix-redef-crash branch Nov 11, 2022
hauntsaninja added a commit that referenced this pull request Nov 11, 2022
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latest mypy 0.990 crashes with SQLAlchemy main
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