Allow/disallow pep561 packages

[emmatyping]

@Alexander255 has requested a flag --pep561-override to allow listing packages as pep 561 compatible when they are not. This got me thinking and I think the best UX for this would be a pair of flags: --allow-installed-package NAME and --disallow-installed-package NAME (or something like that).

The idea is to solve two problems:

Some people publish PEP 561 packages that are either too unstable or flat out broken for some users. This allows the user to disable the package until things are fixed.

At the other end of the spectrum, some packagers are conservative on adding types, and while they exist, they are not public (yet). Some users would still like to use them.

This would eliminate --no-site-packages, which is a rather blunt instrument.

[JukkaL]

I can see how this could be useful., and this shouldn't be hard to implement

Random thoughts:

• What about --use-installed-package X as the option name? The inverse would be --ignore-installed-package X, perhaps. More ideas:
  • --enable-installed-package X and --disable-installed-package X
  • --use-site-package X and --no-site-package X
  • --site-package X and --no-site-package X
  • --installed-package X and --no-installed-package X
• We can start with --use-site-package (or whatever it would be called) and the inverse can be implemented later. The inverse seems less commonly useful.
• Should this work for subpackages? What about things like --use-site-package google.protobuf?

[emmatyping]

I think I like either --enable-installed-packages X or --use-installed-packages X, ideally I think this should be a list of packages, as people may want to allowlist/denylist groups of packages.

[msullivan]

I am a "-0" on this; I don't like adding more complexity to our import system. But also I'd be happy to merge it. Let's make a decision on it so we can resolve #8512.

[emmatyping]

Yeah I'm kinda -0 on this too. The complexity would be unfortunate, but I can kinda see the use case.

[Vozf]

Just to clarify all selected flags will be usable via mypy.ini, correct?

[embray]

👍 for this. I am using some packages that are fully typed and are PEP-561 compatible in principle (they have a py.typed) but due to a packaging bug (third-party package, outside my control) were missing the py.typed file when installed.

I can of course work around it by creating the py.typed file in the site-packages/<package_name> but it would be more convenient if I could tell mypy "yes, this package really does have type hints".