Feature Request: Warn when instance attribute shadows class attribute

[alippai]

I suggest adding a linting rule to warn when an instance attribute shadows a class attribute of the same name. This can help prevent bugs from unintended overriding of class-level variables.

class A:
    x = 1
    def get_x(self):
        return self.x

a = A()
a.x = 2  # Shadows class attribute 'x'
print(a.get_x())  # Outputs 2, but class attribute 'x' is 1

In this example, assigning to a.x creates an instance attribute that overshadows the class attribute x. Accessing self.x in get_x now refers to the instance attribute, which might not be the intended behavior.

Similarly, the subclassing is error prone:

class Parent:
    def __init__(self):
        self.value = 'instance'

    def get_value(self):
        return self.value

class Child(Parent):
    value = 'class'  # Class attribute shadows instance attribute

c = Child()
print(c.get_value())  # Outputs 'instance'

[alippai]

@MichaReiser this might be relevant here: #10891

[MichaReiser]

Thanks for opening this rule idea.

The main challenge with this rule is that it probably requires type inference, or at least more advanced analysis than what Ruff does today to be useful:

• Ruff needs to understand that a is an instance of A
• Ideally, Ruff understands that a is an instance of A even if A is defined in another file or if A is the result of a function call
• Ruff needs to understand whether a.x is an instance attribute or not (I think that's doable because detecting class attributes is "easy enough).