Consider marking `UP026`'s fix unsafe

[robsdedude]

Summary

I'll quote the readme of the mock package:

mock is now part of the Python standard library, available as unittest.mock in Python 3.3 onwards.

This package contains a rolling backport of the standard library mock code compatible with Python 3.6 and up.

So replacing import mock with from unittest import mock is not equivalent given a code-base that's on an old-ish Python version but want to use new features of mock and therefore decides to use the stand-alone package. Arguably, such code-base should be disabling the lint, nonetheless does the fix cause a little mayhem first should the rule be enabled.

[ntBre]

Thanks! This makes some sense to me. Out of curiosity, did you run into any specific APIs that didn't work in unittest.mock? This is probably not too reliable, but I quickly grepped through the unittest.mock docs and most of the version-related changes were for 3.8 or earlier (DEFAULT_TIMEOUT was the one exception I noticed). We could potentially make only fixes touching known-problematic APIs unsafe instead of the whole fix, if it's easy to enumerate them.

[MichaReiser]

I'd prefer if we disable the rule for APIs that we know are unavailable on the target version rather than making the fix unsafe.

[robsdedude]

Iirc, ruff only computes lints on a per-file basis. If that's the case, it's not possible to know where a certain mock object is going to be used. Usually the mock specific methods are being called in the same test function where the mock is being created, but it's entirely possible that a helper method in another file is being used to set up the mock or to perform common assertions. In such cases the suggested approach might still break.

Further, this approach easily ends up with a test suite using different mock versions. Besides differences in the API the versions might also differ in behaviour (think fixes). I personally think making ruff introduce a mix of dependency versions where there (hopefully) was none before is a trip hazard.