@sys.meta_path.append type error

[warsaw]

I'm not sure if this is a bug in Python or in mypy, or there is no bug at all. It seems a bit weird given the contents of sys.meta_path and my grep-fu must suck because I can't find where the signature is defined. Here's the code:

import sys


@sys.meta_path.append
class LostFinder:
    @staticmethod
    def find_spec(*args, **kws):
        return None

and the error:

$ mypy foo.py 
foo.py:4: error: Argument 1 to "append" of "list" has incompatible type "Type[LostFinder]"; expected "MetaPathFinder"

and the contents of sys.meta_path on a vanilla Python:

$ python3.7 -I -c "import sys; print(sys.meta_path)"
[<class '_frozen_importlib.BuiltinImporter'>, <class '_frozen_importlib.FrozenImporter'>, <class '_frozen_importlib_external.PathFinder'>]

[msullivan]

The type of sys.meta_path is defined in the typeshed stub: https://github.com/python/typeshed/blob/master/stdlib/3/sys.pyi#L42

The class is defined here: https://github.com/python/typeshed/blob/master/stdlib/3/importlib/abc.pyi#L55

Subclassing from it should fix the error.

I don't think there is any bug here?

[msullivan]

(Feel free to reopen if you disagree)

[jaraco]

Perhaps it's a bug in typeshed... and the spec should be more lenient, to allow for MetaPathFinder or Type[MetaPathFinder] or maybe any other class that duck types a MetaPathFinder (because even the stdlib <class '_frozen_importlib.BuiltinImporter'> is not an instance of MetaPathFinder.

[msullivan]

Oh, indeed, I missed that it contained classes and not the objects.

1. It is definitely a typeshed bug that it doesn't allow Type[MetaPathFinder]
2. Arguably typeshed should use a protocol too

[ilevkivskyi]

Yes it is a typeshed bug, not mypy bug. @msullivan's suggestion should fix it in typeshed.