Type hint for Distribution.metadata is incorrect

[mpurnell1]

I encountered this while trying to help sentry-python prepare for the changes mentioned in #371. Their linter errored with "PackageMetadata" has no attribute "get", since metadata is advertised to return _meta.PackageMetadata, but actually returns _adapters.Message.

Is this intended, and if so what is the workaroud for the linter?

Reproduction steps:

1. git clone git@github.com:mpurnell1/sentry-python.git
2. pip install tox
3. tox -e linters

[mpurnell1]

It seems the issue is not caused by metadata returning _adapters.Message. Workaround is below, but the typing should maybe still be updated.

... mypy is complaining about PackageMetadata having no .get. Like in most cases that have to do with mypy I'm not sure what's confusing since it isn't the case. Feel free to add a type: ignore[attr-defined] on the line to silence it.

Originally posted by @sentrivana in getsentry/sentry-python#4039 (comment)

[jaraco]

Note that PackageMetadata is a Protocol, which simply defines the interface that's expected for that type. _adapters.Message is the implementation that's expected to implement that protocol. Feel free to read up on protocols and typing systems or ask your colleagues for more details.

And as you can see from the spec, PackageMetadata does in fact implement get, so the problem is likely with the linter (mypy) or with sentry's use. It's also conceivable that you're working with a different, older version of importlib_metadata.

I don't believe there's any problem with importlib_metadata, but if there is, you'll need to dig deeper to characterize the problem in a more minimal reproducer (e.g. can you replicate the issue in a few lines using just mypy and importlib_metadata?).