unreachable code after suppress()

[Dreamsorcerer]

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]

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]

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]

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

[gvanrossum]

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]

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