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@sys.meta_path.append type error #5589
Description
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 Noneand 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'>]
Activity
msullivan commentedon Sep 8, 2018
The type of
sys.meta_pathis defined in the typeshed stub: https://github.com/python/typeshed/blob/master/stdlib/3/sys.pyi#L42The 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 commentedon Sep 8, 2018
(Feel free to reopen if you disagree)
jaraco commentedon Sep 8, 2018
Perhaps it's a bug in typeshed... and the spec should be more lenient, to allow for
MetaPathFinderorType[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 commentedon Sep 8, 2018
Oh, indeed, I missed that it contained classes and not the objects.
Type[MetaPathFinder]ilevkivskyi commentedon Sep 8, 2018
Yes it is a typeshed bug, not mypy bug. @msullivan's suggestion should fix it in typeshed.
Ignore python/mypy#5589