Immutable fields

Project overview and reading path

Several components share feature configuration. One component must not silently change the features seen by another.

Typical Python

from dataclasses import dataclass


@dataclass(frozen=True)
class Config:
    features: list[str]


config = Config(["height", "width"])
config.features.append("area")

Freezing the record blocks field reassignment but permits mutation of the list. The append succeeds and changes shared state.

Alternative

@dataclass(frozen=True)
class Experiment:
    seed: int
    features: tuple[str, ...]


config = Experiment(seed=17, features=("height", "width"))
expanded_features = config.features + ("area",)

Source

The tuple field has no .append, so that operation is rejected as missing-attribute. Frozen fields reject reassignment, and Final protects the default binding in checked code. The caller’s expanded_features is a new tuple; config.features still contains only ("height", "width").

Immutability is shallow. A frozen record containing a mutable array still exposes that array’s mutation. Here each feature name is a string, so a tuple fits the actual contract. Environment parsing is a separate boundary.