198 – Data class with default mutable value

If you try to set a data class field to a mutable value, you get an error:

from dataclasses import dataclass

@dataclass
class Error:
    lineno: int
    error_type: str
    notes: list[str] = []

ValueError: mutable default <class 'list'> for field notes is not allowed: use default_factory

The decorator dataclass validates each field in your data class and it preemptively throws an error when it sees the list [] being used as the default value for the field notes. The reason you get the error is because it is highly likely that your code will have bugs if the error wasn’t raised.

The correct way to have a list be the default value is by using dataclasses.field and the parameter default_factory:

from dataclasses import dataclass, field

@dataclass
class Error:
    lineno: int
    error_type: str
    notes: list[str] = field(
        default_factory=list
    )

The parameter default_factory accepts a zero-argument function that is called whenever you need to populate your field with a default.