Python 3.7+dataclassesfunction

dataclasses.field()

field

The field() function sets extra properties of a field: its default value, its factory, and whether it takes part in repr/compare/init.

Syntax

dataclasses.field(*, default=MISSING, default_factory=MISSING, repr=True, hash=None, compare=True, init=True)

field() exists mainly because of mutable defaults: a value written in the class body would be shared by every instance, so dataclass raises ValueError on an unhashable default and demands default_factory. The catch is that the check keys on hashability — your own mutable class with a normal __hash__ slips through as default= and is silently shared by all instances. compare=False and repr=False are handy for bookkeeping fields such as caches or ids that should not affect equality or output.

Examples

from dataclasses import dataclass, field
@dataclass
class Bag:
    items: list = field(default_factory=list)
    _id: int = field(default=0, repr=False, compare=False)
b = Bag()
b.items.append(1)
repr(b) # → 'Bag(items=[1])'
Bag() == Bag() # → True

See also

Common errors

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