Python Mixin - Unresolved Attribute Reference [PyCharm]

Declare the necessary fields in the Mixin like:

class Mixin:
    foo: str

    def complex_operation(self):
        return self.foo.capitalize() 

This way the mixin actually declares the fields a class must have to be able to use this mixin. Type hint will create warnings if extending class will put incompatible type into declared field.

edit: Replaced foo = None with foo:str as suggested by @valex

I see few options.

1) Type annotations (i think this is cleanest solution):

class Mixin:
    foo: str

    def complex_operation(self):
        return self.foo.capitalize()

2) Default None (@ikamen option):

class Mixin:
    foo = None

    def complex_operation(self):
        return self.foo.capitalize()

3) Suppress unresolved reference error for class or for specific line (i think this is more dirty way than first two):

# noinspection PyUnresolvedReferences
class Mixin:
    def complex_operation(self):
        return self.foo.capitalize()

class Mixin:
    def complex_operation(self):
        # noinspection PyUnresolvedReferences
        return self.foo.capitalize()