false unreachable error after pytest.raises block

[rkm]

Hi. mypy returns an unreachable error for code after a pytets.raises block:

$ cat t.py
import pytest

def testeroni() -> None:
    with pytest.raises(Exception):
        raise Exception("oh noes")
    print("hello there")

$ mypy --version
mypy 0.780

$ mypy t.py
t.py: note: In function "testeroni":
t.py:7: error: Statement is unreachable
        print("hello there")
        ^
Found 1 error in 1 file (checked 1 source file)

I'm actually not sure if this is a bug in mypy, since it might not be aware that pytest swallows the exception there. Any advice appreciated!

[JelleZijlstra]

Mypy by default assumes context managers don't suppress exceptions, because that's more common in general. See https://github.com/python/typeshed/blob/master/CONTRIBUTING.md#conventions (second to last paragraph) and #7214 (comment).

To fix this issue pytest.raises should be annotated as a ContextManager that returns bool.

[emmatyping]

@JelleZijlstra yeah, this is unfortunate though, because for an un-annotated library (like pytest) there is no way for a user to change mypy's behavior here. Perhaps we should default to context managers that are Any can swallow exceptions?

[JelleZijlstra]

@ethanhs that would also cause false positives (about missing return statements).

[emmatyping]

@JelleZijlstra yes but those false positives can be silenced via asserts. I think it is better to have false positives that can be disabled. Furthermore, incorrectly calling a branch dead could lead to false negatives.

[bluetech]

pytest 6 is going to have published type annotations, hopefully (link), though we're still working on the details.

[rkm]

I'm happy to confirm that this is fixed with pytest-dev/pytest#7448. Thanks again for your work on this @bluetech! :)