mutable-class-default (RUF012)

What it does

Checks for mutable default values in class attributes.

Why is this bad?

Mutable default values share state across all instances of the class, while not being obvious. This can lead to bugs when the attributes are changed in one instance, as those changes will unexpectedly affect all other instances.

Generally speaking, you probably want to avoid having mutable default values in the class body at all; instead, these variables should usually be initialized in __init__. However, other possible fixes for the issue can include:

• Explicitly annotating the variable with typing.ClassVar to indicate that it is intended to be shared across all instances.
• Using an immutable data type (e.g. a tuple instead of a list) for the default value.

Example

class A:
    variable_1: list[int] = []
    variable_2: set[int] = set()
    variable_3: dict[str, int] = {}

Use instead:

class A:
    def __init__(self) -> None:
        self.variable_1: list[int] = []
        self.variable_2: set[int] = set()
        self.variable_3: dict[str, int] = {}

Or:

from typing import ClassVar


class A:
    variable_1: ClassVar[list[int]] = []
    variable_2: ClassVar[set[int]] = set()
    variable_3: ClassVar[dict[str, int]] = {}

Or:

class A:
    variable_1: list[int] | None = None
    variable_2: set[int] | None = None
    variable_3: dict[str, int] | None = None

Or:

from collections.abc import Sequence, Mapping, Set as AbstractSet
from types import MappingProxyType


class A:
    variable_1: Sequence[int] = ()
    variable_2: AbstractSet[int] = frozenset()
    variable_3: Mapping[str, int] = MappingProxyType({})