Invalid Attribute Access

What it does

Checks for assignments to class variables from instances and assignments to instance-only attributes from their class. Also checks for reads and writes of generic instance attributes through a generic class or a specialized generic alias.

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

A generic instance attribute has a type that depends on the class's type parameters. Specializing a generic class does not create separate class attribute storage, so these attributes cannot be accessed through the generic class or a specialized alias. Access through a type[...] receiver is allowed because it can refer to a concrete subclass with its own class attributes.

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

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

from typing import Generic, TypeVar

T = TypeVar("T")


class Box(Generic[T]):
    value: T


Box[int].value = 1  # error
Box.value  # error

box = Box[int]()
box.value = 1  # okay