Disallow access to instance variable via class

[JukkaL]

Currently an instance variable can be accessed using the class, which is wrong. For example, this code is accepted by the type checker:

class A:
    def f(self) -> None:
        self.x = 1
A.x = 1 # accepted, but should be an error

Class variables need to be assigned in the class body:

class A:
    x = 1
A.x = 1 # ok, and should be ok

[JukkaL]

Actually, I'm no longer convinced that this is a bug. Turning this into an enhancement proposal.

[gvanrossum]

Yeah, this may just be a somewhat unconventional wy of setting the default.

[JukkaL]

Postponing this as I'm not sure if this is a good idea but might still be worth considering.

[mark-kubacki]

The OP has it as writing—I ran into this erroneously reading a variable; consider:

from __future__ import annotations
import logging

from typing import Type


class A:
    def __init__(self):
        self._log = logging.getLogger()
        pass

    @classmethod
    def from_spam(cls: Type[A], spam):
        if spam:
            cls._log.warning("got spam")  # accepted, but should be an error
        return cls()

[gsakkis]

I ran into a situation which is almost the opposite problem of the original one: not only a given attribute is deliberately accessible by both the instance and the class, but the type in each case different. Unsurprisingly mypy complains about Incompatible types in assignment.

Minimal example:

class A:
    x = "a"

    def __init__(self) -> None:
        self.x = 1

assert A().x == 1
assert A.x == "a"

I tried annotating x both as an instance and class variable but this caused a Name "x" already defined error without getting rid of the first one:

class A:
    x: int
    x: ClassVar[str] = "a"

    def __init__(self) -> None:
        self.x = 1