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unreachable code after suppress() #9199

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Dreamsorcerer opened this issue Jul 24, 2020 · 5 comments
Closed

unreachable code after suppress() #9199

Dreamsorcerer opened this issue Jul 24, 2020 · 5 comments

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@Dreamsorcerer
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Mypy reports this as unreachable code on the last line:

with suppress(KeyError):
    return _get_session()[name]

weights = AB_TESTS.get(name)

If the indexing causes a KeyError, this code will be reached. _get_session() is Any, but I also tried casting it to a Dict. So, it looks like Mypy assumes this will always succeed, despite us explicitly catching the error.
@ilevkivskyi
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IIRC there is a way to tell mypy that a context manager can swallow an exception by annotating its __exit__() method in a special way, so this may be fixed by updating suppress.__exit__() annotation in typeshed. I however don't remember details on this.

@Dreamsorcerer
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It appears that's already been done (by return bool): https://github.com/python/typeshed/pull/3179/files

I think the problem is that Mypy fails to recognise that if an exception is thrown then the return won't be executed, and therefore there are 2 branches of code here.

@Dreamsorcerer
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Actually, it looks like the exact same issue that introduced this: #7214
So, unsure why this isn't working.

@gvanrossum
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Perhaps you have some configuration flag that affects this? And (just to be sure) what mypy version are you using? I can't repro your issue, but it looks like you didn't show all the code of a self-contained example.

@Dreamsorcerer
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Hmm, running it again, the error has now disappeared...
I'll close this unless I see it return again.

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@Dreamsorcerer @gvanrossum @ilevkivskyi