No overload variant of "fromkeys" of "dict" matches argument types [typing.Iterable[Any], Union[Any, def (*Any, **Any) -> Any]]

[trnsnt]

Hello,

I have defined a dict wrapper where I override fromkeys method.
I have this method signature

    def fromkeys(cls, iterable, value=None):
        # type: (Iterable, Union[Any, Callable]) -> DictWrapper

and this give me the following error : No overload variant of "fromkeys" of "dict" matches argument types [typing.Iterable[Any], Union[Any, def (_Any, *_Any) -> Any]]

Thanks !

[gvanrossum]

Thanks for the question! I'd be happy to help but I'm not sure I understand the context of that definition. Can you show a complete, small, self-contained program that exhibits this behavior?

[trnsnt]

Hello, thanks for the reply. Here is what I have done.
I have this class

class DictWrapper(dict):

    @classmethod
    def fromkeys(cls, iterable, value=None):
        # type: (Iterable, Union[Any, Callable]) -> DictWrapper
        if not callable(value):
            return DictWrapper(super(d, cls).fromkeys(iterable, value))

        def _():
            for key in iterable:
                yield key, value(key)

        return DictWrapper(_())

Code is working fine (Python 3.5), but when I run mypy, I have the following error :

No overload variant of "fromkeys" of "dict" matches argument types [typing.Iterable[Any], Union[Any, def (Any, *Any) -> Any]]

I am not sure if it is a problem, or if I have done something wrong.

[gvanrossum]

Oh, it seems you've hit two issues! When I run that example I get:

__tmp__.py: note: In class "DictWrapper":
__tmp__.py:6: error: Signature of "fromkeys" incompatible with supertype "dict"
__tmp__.py: note: In member "fromkeys" of class "DictWrapper":
__tmp__.py:9: error: No overload variant of "fromkeys" of "dict" matches argument types [typing.Iterable[Any], Union[Any, def (*Any, **Any) -> Any]]

(Line 6 is the def, line 9 is the super() call.)

The first complaint is because in typeshed, fromkeys() is defined as a static method, while it is actually a class method. I don't know why this is, but I think I can just fix that.

The second complaint is probably because mypy doesn't understand callable(value) and hence it doesn't believe that the value you pass to the superclass version of fromkeys() matches the required type. This is issue #1973.

My recommendation is to add a # type: ignore to the offending line and proceed.

[rwbarton]

Isn't the second error just because you passed an Iterable, but dict.fromkeys expects a Sequence?

[gvanrossum]

Oh, good catch! I will fix that at the same time. Wait for the next iteration of python/typeshed#604.

[gvanrossum]

OK then I think I have fixed both issues and this can be closed. @trnsnt, to get the fix right away you'll have to pull down the mypy repo, git submodule update, and then in the typeshed subdir do something like "git co master; git pull".

[gvanrossum]

Sadly making fromkeys() a classmethod breaks, due to #328, so this is not fixed. Really sorry!! I went too fast here.