[Spec] Modifying a `ClassVar` on an instance

Typing
1

According to the typing spec:

The typing.ClassVar type qualifier is used to annotate class variables that should not be set on class instances.

However, what about modifying the ClassVar with __setitem__ or __setattr__ on an instance? It feels like this should also not be allowed. Two examples:

__setitem__ example: pyright playground, mypy playground, ty playground

from typing import ClassVar

class Foo:
    d: ClassVar[dict[str, object]] = {}

    def set_value(self, key: str, value: object) -> None:
        self.d[key] = value  # <-- Use Foo.d[key] = value instead?!

__setattr__ example: pyright playground, mypy playground, ty playground

from typing import ClassVar
from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float

class Bar:
    p: ClassVar[Point] = Point(0.0, 0.0)

    def set_x(self, value: float) -> None:
        self.p.x = value  # <-- Use Bar.p.x = value instead?!

Enforcing that setting attributes/items should only be done on the class and not the instance level would be a good reminder that such modification does indeed apply to all instances.

1 Like
2

It’s more intention revealing to write it your way. And even to refactor both methods into classmethods. It’d be a great optional rule for a linter to warn about. And for a different ClassOnlyVar typequalifier.

But this would prohibit combining classes with ClassVars marked with mixins that use those class vars. A lot of extensible code will deliberately refer to an class variable on self, to let a subclass override it with an instance variable. It might also wish to use ClassVar, simply to prevent accidental reassignment.

3

Do you have an example? Note that the idea of this proposal is to flag

self.class_var.x.y.z[...] = ...
self.class_var.x.y.z = ...

Simply accessing/reading the ClassVar via self.class_var is not affected in any way.
(Basically, ClassVars that are accessed on an instance should be treated as immutable).

4

I believe networkx used to (and probably still does) do this. I agree with you however, and think it’s simply poor design.