Mypy should look in metaclass for operations on class objects

Mypy complains about this line:

StringWrapper >> "foo"

With:

Unsupported left operand type for >> ("StringWrapper")

However, __rshift__ is defined on StringWrapper AND on StringWrapper's metaclass.

I understand the notation seems strange, calling shift on a class, but I'm making a library in which this API makes sense. The related code works, and applying >> does call __rshift__ on the metaclass. The only problem is mypy mentioning it. I can force ignore it, but I wish that dev using my lib won't have to do so.

Does this work as expected when the left arg is not a class? I suspect mypy is treating classes specially somehow and not looking at the metaclass.

Just tried, indeed with StringWrapper() >> "foo", no more warning.

OK, I've changed the title accordingly (I don't think it's specific to >>).

Related: #2392

A large source of "errors" for me is using metaclasses for class properties, here's a minimal example:

class Meta(type):
    @property
    def prop(cls):
        return "Hi!"

class Concrete(object, metaclass=Meta):
    pass

print(Concrete.prop)

I suspect it'll apply for anything to do with metaclasses though (as Guido suggested), given the two examples here are pretty disparate.

Yep it's just metaclass stuff in general

class Meta(type):
    attr = "attr"

    def fn(cls):
        return "fn"

class Concrete(object, metaclass=Meta):
    pass

print(Concrete.fn())
print(Concrete.attr)

It looks like this issue is now fixed by #2475

The property, attribute and method examples now work.

__rshift__ still doesn't seem to work:

class Meta(type):
    def __rshift__(self, x):
        return 'foo'

class Concrete(object, metaclass=Meta):
    pass

print(Concrete >> 1)  # Unsupported operand types for >> ("Concrete" and "int")

@JukkaL

@JukkaL Hm, this is very strange, I tried this:

class Meta(type):
    def __rshift__(self, x):
        ...

class Concrete(metaclass=Meta):
    pass

print(Concrete >> 'foo')

And it works (note that I use a string, like in OP example, not an integer).

I can confirm that this behaves differently for int and str operands. It may have something to do with integers also defining __rshift__ and friends.

~~Yes, it's failing for all types but the reverse method hides it for int. Maybe the lookup in has_member should be different.~~

(Sorry, scratch the above: more likely is that the reverse method prohibits the lookup)

Check for member in type-fallback (#2822)

#2822 fixed the remaining issue.

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Mypy should look in metaclass for operations on class objects #2413

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Mypy should look in metaclass for operations on class objects #2413

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gvanrossum commented on Nov 7, 2016

sametmax commented on Nov 7, 2016

gvanrossum commented on Nov 7, 2016

elazarg commented on Nov 12, 2016

TRManderson commented on Nov 18, 2016

TRManderson commented on Nov 18, 2016

ilevkivskyi commented on Feb 7, 2017

JukkaL commented on Feb 7, 2017

ilevkivskyi commented on Feb 7, 2017

JukkaL commented on Feb 7, 2017

elazarg commented on Feb 7, 2017

elazarg commented on Feb 7, 2017

JukkaL commented on Feb 7, 2017

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