Add support for type checking with mypy plugins

[flaeppe]

~I'm not totally sure if this approach always works or if there's stuff that I'm missing, but it seems to work. I'll try to explain myself here.~

~The change utilises pip's --target(docs) argument to point out where to install a project's additional dependencies, passed in via pip_cmd. This replaces the virtual env creation for each project, since that shouldn't be necessary (I think?).~

~In addition to what's mentioned above~ we start setting the PYTHONPATH env variable(docs), to point out an additional search path for module files, namely the site-packages directory ~with the additional dependencies~ inside the project's virtualenv.

That should allow discovery of project dependencies while keeping the installed mypy versions independent and reusable.

• Closes #8
• Closes #102

Outdated

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All this isn't included, but I was thinking that it should be possible to use pip install --target for both mypy installations as well, instead of a virtual env. That leads to having 1 python/pip executable, 1 directory for new_mypy, 1 directory for old_mypy and then all project dependency installations. Although I'm unsure what --python-executable= does and if it could be affected. I'm also not sure how to handle compiled mypy.

Essentially what'll happen with the above changes is that we'll have a directory structure like e.g.

repo-dir/
├── new_mypy
│   └── bin
│       └── mypy
├── old_mypy
│   └── bin
│       └── mypy
├── projects
│   ├── A
│   └── B
└── python
    └── bin
        └── python

We run mypy with something corresponding to

PYTHONPATH="repo-dir/new_mypy:repo-dir/projects/A" repo-dir/new_mypy/bin/mypy --python-executable=repo-dir/python/bin/python ...
PYTHONPATH="repo-dir/old_mypy:repo-dir/projects/A" repo-dir/old_mypy/bin/mypy --python-executable=repo-dir/python/bin/python ...

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Here's some example debug output:

...
/tmp/mypy_primer/projects/_django-choicefield_venv/bin/pip install . django-stubs pytest         in /tmp/mypy_primer/projects/django-choicefield
PYTHONPATH=/tmp/mypy_primer/new_mypy/venv/lib/python3.12/site-packages:/tmp/mypy_primer/projects/_django-choicefield_venv/lib/python3.12/site-packages
/tmp/mypy_primer/new_mypy/venv/bin/mypy --python-executable=/tmp/mypy_primer/projects/_django-choicefield_venv/bin/python --warn-unused-ignores --warn-redundant-casts --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit=-1     in /tmp/mypy_primer/projects/django-choicefield
PYTHONPATH=/tmp/mypy_primer/old_mypy/venv/lib/python3.12/site-packages:/tmp/mypy_primer/projects/_django-choicefield_venv/lib/python3.12/site-packages
/tmp/mypy_primer/old_mypy/venv/bin/mypy --python-executable=/tmp/mypy_primer/projects/_django-choicefield_venv/bin/python --warn-unused-ignores --warn-redundant-casts --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit=-1     in /tmp/mypy_primer/projects/django-choicefield
/tmp/mypy_primer/new_mypy/venv/bin/mypy on django-choicefield took 4.96s
/tmp/mypy_primer/old_mypy/venv/bin/mypy on django-choicefield took 4.98s
...