182 – Field default factory

Default mutable values create all kinds of problems, so using a mutable value as a default for a data class attribute raises an error:

from dataclasses import dataclass

@dataclass
class Exercise:
    name: str
    reps: int
    notes: list[str] = []  # Wrong!
# ValueError

The correct way to set the attribute notes to have an empty list by default is by using dataclasses.field:

from dataclasses import dataclass, field

@dataclass
class Exercise:
    name: str
    reps: int
    notes: list[str] = field(  # Right!
        default_factory=list
    )

The parameter default_factory can be set to a callable that should take no arguments. The callable is called each time the default value is needed.