Stub for `importlib.metadata` on Python 3.10 is incomplete

[Molkree]

• entry_points() was changed in 3.10
• metadata() function returns PackageMetadata which got new attribute json

[hauntsaninja]

Thanks, PR would be appreciated (feel free to fix both in a single PR)

[itaisteinherz]

Wasn't the second item resolved in #6557?

[Molkree]

Oh, that's interesting. I see PackageMetadata class being added with json attribute but metadata() still returns Message and not the new structure (on Python 3.10+)?

typeshed/stdlib/importlib/metadata/__init__.pyi

Line 112 in 4343153

def metadata(distribution_name: str) -> Message: ...

[itaisteinherz]

It seems like the typings in typeshed are inaccurate, however mypy is still able to tell that the return type of metadata() is PackageMetadata using the type hints in CPython:

https://github.com/python/cpython/blob/b77158b4da449ec5b8f682816a79d004fd65ed07/Lib/importlib/metadata/__init__.py#L954

We should probably update the typings here to match those in CPython. I'll open a PR.

[hauntsaninja]

(mypy and other type checkers never look at type hints in CPython code)

[itaisteinherz]

(mypy and other type checkers never look at type hints in CPython code)

Then how come mypy reports different types than the ones in typeshed?

[hauntsaninja]

What exactly is the difference you're seeing? (sorry, haven't been following discussion here too closely)

mypy vendors a copy of typeshed. So the types you get depend on what version of mypy you're using and what types were vendored when that version was released. You can use mypy --custom-typeshed-dir ~/dev/my-local-typeshed-checkout repro.py to check something using your local typeshed checkout.

[itaisteinherz]

I ran mypy with a local clone of typeshed (on the latest commit in master) on this snippet:

from importlib_metadata import metadata
m = metadata("mypy")
reveal_type(m)

And received the following output:

scratch.py:3: note: Revealed type is "importlib_metadata._meta.PackageMetadata"

Which doesn't make much sense to me, as:

typeshed/stdlib/importlib/metadata/__init__.pyi

Line 140 in 7b024f5

def metadata(distribution_name: str) -> Message: ...

[hauntsaninja]

importlib_metadata is the backport, which is py.typed. Typeshed provides stubs for importlib.metadata, which is in the standard library.

[AlexWaygood]

@hauntsaninja beat me to it while I was halfway through writing this, but:

importlib_metadata is a different module to importlib.metadata:

• importlib.metadata is a standard-library module. For the standard library, mypy looks at typeshed stubs for its type annotations, because most of the standard library does not have inline type annotations.
• importlib_metadata is a third-party package on PyPI, which serves as a backport of importlib.metadata to earlier versions of Python. Because it has a py.typed file, mypy looks at the inline annotations in the source code of the library. rather than a stub file like those provided by typeshed.

This issue is about the stub file for the importlib.metadata standard-library module.

[itaisteinherz]

@hauntsaninja @AlexWaygood Thanks for explaining :)