Semimutable

A dataclass drop-in that allows you to define individual fields as immutable.

Usage

Simply replace all your dataclasses imports with semimutable, and use field(frozen=True) for the fields you want to be immutable.

field takes in the same parameters as dataclasses.field, with an extra frozen boolean flag that defaults to False. You can specify default values, default factories, and other options just like you would with a regular dataclass field.

Here is one example from our tests.

from semimutable import dataclass, field

@dataclass
class Simple:
    x: int = field(frozen=True)
    y: int = 0 # normal, mutable field

def test_frozen_field_is_immutable():
    obj = Simple(x=1, y=2)
    with pytest.raises(TypeError):
        obj.x = 99

def test_non_frozen_field_is_mutable():
    obj = Simple(x=1, y=2)
    obj.y = 42
    assert obj.y == 42

Credits

Parts of this library are derived from Python's standard library dataclasses module. The original implementation is distributed under the Python Software Foundation License. See LICENSE.PSF for the full license text.