False positive "Missing return statement" when exception is raised from function in try-except block

[jamesbraza]

I am reporting what I think is a bug in mypy. I think it raises a false positive error: Missing return statement when one has a function that is guaranteed to raise an exception (never return) while in the except clause of a try statement.

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Code Sample

false_pos_no_return.py:

def raise_exception():
    raise Exception


def test() -> int:
    try:
        return 1
    except Exception:
        raise_exception()

What mypy reports:

path/to/false_pos_no_return.py: note: In function "test":
path/to/false_pos_no_return.py:5: error: Missing return statement  [return]

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Workarounds

One can add either of the following statements at the end of the except clause:

• raise
• assert False

And the error goes away: Success: no issues found in 1 source file

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Expectations

My expectations are that mypy should be able to detect that raise_exception will never actually return.

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Similar Issues

#1229 seems to be similar, but it seems a decisive conclusion wasn't reached.

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Versions

python==3.6.5
mypy==0.780

[JelleZijlstra]

You should annotate raise_exception() as returning typing.NoReturn. By default, mypy assumes all functions return something.

[jamesbraza]

Ah yes, that does it! Thank you again @JelleZijlstra!

[andredasilvapinto]

what if raise_exception() lives in an external library that you cannot change?

[JelleZijlstra]

@andredasilvapinto you could provide a stub file for the library, or write something like assert False, "unreachable" to help mypy.

[jamesbraza]

I believe the answer in that case is to make a stub, read here. I am not sure how to make an in-module stub. If you figure it out, I would be interested to see the syntax.

To build on the assert False, "unreachable" method, you could also use typing.cast. See here:

from typing import Callable, NoReturn, cast

from .third_party_lib import raise_exception  # Return type is not type hinted

def test() -> int:
    try:
        return 1
    except Exception:
        # raise_exception()  # mypy error
        cast(Callable[[], NoReturn], raise_exception)()  # mypy success
        # assert False, "unreachable"  # mypy success

if __name__ == "__main__":
    test()

[theRealProHacker]

@jamesbraza Maybe even better is making the cast globally, if possible:

from typing import Callable, NoReturn, cast
from .third_party_lib import raise_exception

raise_exception = cast(Callable[[], NoReturn], raise_exception)

Then you can use it anywhere and don't need to copy the cast everywhere it is used. Alternatively, you could also rename the imported function to avoid redefining the type of raise_exception:

from typing import Callable, NoReturn, cast
from .third_party_lib import raise_exception as _raise_exception

raise_exception = cast(Callable[[], NoReturn], _raise_exception)
del _raise_exception # optional