Add support for mypy

[Hnasar]

It looks like mypy has support for outputting Cobertura reports:
https://github.com/python/mypy/blob/master/mypy/report.py#L481

Maybe it'd be trivial to add support to mypy for diff-cover and diff-quality?

[Bachmann1234]

If Mypy outputs Cobertura then you can already use diff-cover! You just use that mypy coverage file. In theory I think it would all just work

[blueyed]

Yeah, it should work already. Working on this for pytest in pytest-dev/pytest#6486.
This issue can be closed.

[blueyed]

Maybe adding support for --violations=mypy might make sense though (https://github.com/Bachmann1234/diff_cover#adding-diff-quality-support-for-a-new-quality-checker), but that would have to be done for mypy itself then.

[m-aciek]

Mypy's Cobertura report is lackonic. It treats lines that generate warning or error as "uncovered", but doesn't provide a cause. The sample output from a run is:

% mypy … --cobertura-xml-report .
[all violations printed]
Generated Cobertura report: […]/cobertura.xml
Found 38 errors in 1 file (checked 1 source file)
% uvx diff-cover cobertura.xml --compare-branch master
-------------
Diff Coverage
Diff: master...HEAD, staged and unstaged changes
-------------
[…].py (50.0%): Missing lines 101,108-109,140,172,175
-------------
Total:   12 lines
Missing: 6 lines
Coverage: 50%
-------------

It's definitely worth to integrate mypy, that would allow to see the violations content.

[YeeaaahMan]

I've prepared a PR here: /pull/540