Freeze Value Objects

Chapter 7 made hash inputs read-only with properties. A dataclass can express the same decision directly.

Freeze a value

Assignment is refused:

points.amount = 5

The amount attribute cannot be reassigned. It holds an immutable integer, so the value used by equality stays stable and the generated hash is safe:

Entities make a different promise

An attempt exists in order to change. Keep it mutable. It also represents one event, so two matching attempts must retain identity equality:

@dataclass(eq=False)
class QuizAttempt:
    learner: str
    responses: list = field(default_factory=list)

eq=False turns off generated value equality. Ordinary identity equality and identity hashing remain.

Frozen is shallow: it prevents assigning an attribute, but a mutable list stored inside can still change. A genuinely immutable value should itself contain immutable values.

The exercise classifies three values and one entity. Making new versions of frozen values is the next lesson’s single subject.