Disallow access to instance variable via class

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

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

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

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

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()

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

Hello!
2024 here! What are the news since 2013?!

With Python 3.12.6 and mypy 1.11.2, mypy says still nothing on this case, but Pylint and Pylance do it, and there is an error during the code execution.

Plus, for this close similar case:

class A:
    id: Final[int] = 5

    def __init__(self):
        self.id = 10

a = A()

print(A.id) # => 5
print(a.id) # => 10
print(a.__class__.id) # => 5

mypy says Cannot assign to final attribute "id" mypy(misc) on the self.id = 10 line, which is incorrect because self.id defined in the constructor is considered as an instance attribute by Python, and not the class attribute A.id. :/
Pylance does exactly the same mistake. >_< !!!

Prohibit access via class for instance-only attributes (python#20855)

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Disallow access to instance variable via class #240

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Disallow access to instance variable via class #240

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JukkaL commented on May 17, 2015

gvanrossum commented on May 17, 2015

JukkaL commented on Oct 13, 2015

mark-kubacki commented on Feb 18, 2022

gsakkis commented on Sep 11, 2023

vtgn commented on Oct 15, 2024

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