Support negative patterns in python_files option.

[ktbarrett]

What's the problem this feature will solve?

This feature will allow users to create exceptions to python files to look for tests or magic like assertion rewriting is applied to.

Describe the solution you'd like

Being able to specify negative patterns in the python_files option such as !my/**/pattern/*.py or similar.

Alternative Solutions

???

Additional context

Would allow workarounds to #10845, scipy/scipy#22236, and cocotb/cocotb#4343.

[The-Compiler]

There is already an ignore option and the norecursedirs setting. For the scenario where python_files works as intended (selecting which modules in the current directory should be picked up as tests), I don't think pytest needs yet another way to do the same thing.

However, I'm surprised that setting python_files = *.py causes pytest to apply assertion rewriting to all Python files, even outside the current directory. That feels like a bug to me, I'll comment in #10845 about that part.

[ktbarrett]

I'm guessing ignore takes precedence over (applied after) python_files? That should work.

A meta-comment... I've seen several projects make this same mistake of separating inclusions and exclusions into two different variables rather than using one, supporting negative patterns, and having a series of patterns apply in order. IIRC I remember myself being a part of this same issue on coverage. It always ends up becoming a sore point and maintenance effort.

However, I'm surprised that setting python_files = *.py causes pytest to apply assertion rewriting to all Python files, even outside the current directory.

That's desired behavior.

[The-Compiler]

I'm guessing ignore takes precedence over (applied after) python_files? That should work.

I believe the assertion rewriting doesn't take ignore into account at all.

That's desired behavior.

Can you elaborate? I don't think it is, as it's both undocumented and surprising.