Invalid Attribute Access - Ruff

What it does

Checks for assignments to class variables from instances and assignments to instance-only attributes from their class.

An "instance-only" variable is one which is only ever assigned to or declared when accessed via self in an instance method.

Why is this bad?

Incorrect assignments break the rules of the type system and weaken a type checker's ability to accurately reason about your code.

Examples

python

from typing import ClassVar


class C:
    instance_var: int
    class_var: ClassVar[int] = 1

    def __init__(self):
        # instance variable declared in the class body
        self.instance_var = 42

        # instance-only variable not declared in the class body
        self.instance_only_var: int = 42


C.class_var = 3  # okay

C.instance_var = 56  # okay
C().instance_var = 72  # okay

C().instance_only_var = 100  # okay

# Cannot assign to class variable from instance
C().class_var = 3  # error

# Cannot assign to instance-only variable from class
C.instance_only_var = 56  # error