Python Mixin - Unresolved Attribute Reference [PyCharm]

Question

I am using a mixin to separate a range of functionality to a different class. This Mixin is only supposed to be mixable with the only child class:

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

class A(Mixin):
    def __init__(self):
        self.foo = 'foo'

in my method Mixin.complex_operation PyCharm gives warning 'Unresolved Attribute Reference foo'.

Am I using the mixin pattern correctly? Is there a better way? (I would like to have type hints and autocompletion in my mixins, and I would like to have multiple mixins.)

Answer 1

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

Answer 2

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()

Answer 3

So just to compiling my thoughts from the comments for everyone else: The problem is keeping the two classes intrinsically connected while separating functionality. Here are my solutions:

1) Make a module

Have another file, say mixin.py, that has complex_operation as a function. Instead of accepting self as a parameter, have it take a string:

# mixin.py

def complex_operation (foo: str) -> str: return foo.capitalize()

# main.py

from ai import complex_operation
class A: 
    def __init__(self): self.foo = "foo"
print (complex_operation (A().foo))

2) Make a class to accept another class as a parameter

In Mixin's __init__ function, add a parameter to accept an A, and then use that in its methods:

# mixin.py

class Mixin: 
    def __init__(self, a: A): self.a = a
    def complex_operation(self): return self.a.foo.capitalize()

# main.py

from mixin import Mixin
class A:
    def __init__(self): self.foo = "foo"

print (Mixin (A()).complex_operation())