super().__init__(....) in mixins fails with Too many arguments for "__init__" of "object"

[kurtgn]

When I create a mixin class that extends the logic of __init__, the regular thing to do is:

class ExtraValuemixin:

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # some extra initialization
        

However this looks wrong to mypy, as it says:

Too many arguments for "__init__" of "object"

I get it, there's no *args or **kwargs in the object's constructor signature; but this is a mixin, and it relies on its childen's constructors. Ho do I make mypy understand this?

Full example:

class ExtraValuemixin:

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # some extra initialization
        self._extra_value = 1

    def retrieve_extra_value(self):
        return self._extra_value


class ParentObj:

    def __init__(self, value):
        self.value = value


class ChildObj(ExtraValuemixin, ParentObj):
    pass


obj = ChildObj(value=5)

print(obj.retrieve_extra_value())

[mathfac]

Faced with the same issue.

[JukkaL]

Mypy shouldn't complain about this idiom, but I'm not sure what exact rules should be. Maybe calls to super().__init__ should be special cased.

[gvanrossum]

There are lots of problems with mixins. Maybe we should have a way to flag them?

[ilevkivskyi]

@gvanrossum

There are lots of problems with mixins. Maybe we should have a way to flag them?

There is a proposal python/typing#246 (partially based on python/typing#241)

[gvanrossum]

Hmm... Maybe it’s time to work on both of these.

[ilevkivskyi]

Hmm... Maybe it’s time to work on both of these.

It would be great to do it this year, I am not sure however we will have time to do this soon. Since it may be a relatively time-consuming change (taking into account necessary design discussions).

[NeilGirdhar]

This doesn't look like it's just a problem with mixins. This looks like a special case of: #4001 and looks related to #4997

[ZdenekM]

It would be great if I could somehow specify, with what classes the mixin is going to be mixed up. Then, mypy might be able to judge calls to parent methods (including __init__) properly.

[kaushikcfd]

On mypy==0.920, the provided MFE doesn't fail for me. We should probably close this issue?

[LefterisJP]

Going from mypy==0.910 to mypy=0.930 this seems to be fixed. Got bunch of type:ignore commments not needed. So I would say this issue is fixed 😄

[erictraut]

Yes, I can confirm this appears to be fixed in the latest release.

[marcinocto]

I'm still getting error: Too many arguments for "__init__" of "object" in a scenario where one of the arguments is verbose with mypy==0.931

class ExtraValuemixin:
    def __init__(self, value, *args, **kwargs):
        super().__init__(value, *args, **kwargs)

    def retrieve_extra_value(self):
        return self._extra_value


class ParentObj:
    def __init__(self, value):
        self.value = value


class ChildObj(ExtraValuemixin, ParentObj):
    pass


obj = ChildObj(value=5)

print(obj.retrieve_extra_value())

My usecase is adding mixins to classes extending Exception so I know that the message is always there - with simplified code

class CustomExceptionMixin:
    message = ""

    def __init__(
        self, message: str, *args, **kwargs
    ) -> None:
        self.message = message
        super().__init__(message, **kwargs)  # type: ignore