Add `--ignore` flag to `pip check`

What’s the problem this feature will solve?

We currently use pip check quite a bit in conda-forge to make sure that the dependency specifications that the dependencies that we have are correct.

However, some packages pip sometimes specifies as dependencies can also be proper “system”-packages which do not register as pip/python packages. That makes it hard to run pip check because of these “false negatives”. Some examples are cmake, ninja, and likely some others.

Describe the solution you’d like

If we could add a --ignore flag and a corresponding environment variable that could help us quite a bit.

Alternative Solutions

Another solution would be to make pip check check if e.g. the ninja binary is on the $PATH – but that’s brittle. Another solution would be to register system packages as pip installed packages but that’s also not a proper solution.

Additional context

https://github.com/FFY00/meson-python/issues/60

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[pradyunsg]

This seems reasonable! A PR implementing this will be welcome. 😃

[q0w]

if you want env PIP_CHECK_IGNORE_PACKAGES, then flag should be --check-ignore-packages (like pip check --check-ignore-packages) because pip automatically converts all flags to env variables.