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post-init-default (RUF033)

Fix is sometimes available.

What it does

Checks for __post_init__ dataclass methods with parameter defaults.

Why is this bad?

Adding a default value to a parameter in a __post_init__ method has no impact on whether the parameter will have a default value in the dataclass's generated __init__ method. To create an init-only dataclass parameter with a default value, you should use an InitVar field in the dataclass's class body and give that InitVar field a default value.

As the documentation states:

Init-only fields are added as parameters to the generated __init__() method, and are passed to the optional __post_init__() method. They are not otherwise used by dataclasses.

Example

from dataclasses import InitVar, dataclass


@dataclass
class Foo:
    bar: InitVar[int] = 0

    def __post_init__(self, bar: int = 1, baz: int = 2) -> None:
        print(bar, baz)


foo = Foo()  # Prints '0 2'.

Use instead:

from dataclasses import InitVar, dataclass


@dataclass
class Foo:
    bar: InitVar[int] = 1
    baz: InitVar[int] = 2

    def __post_init__(self, bar: int, baz: int) -> None:
        print(bar, baz)


foo = Foo()  # Prints '1 2'.

References