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Domain Modeling · interview question

If you wrote a test around invariants in __post_init__, what should the test prove first?

Multiple choiceMediumchapter-7-curatedpremiumdomain-modelinginvariants-in-post-init
python
from dataclasses import dataclass

@dataclass
class OrderLine:
    sku: str
    quantity: int

    def __post_init__(self) -> None:
        if self.quantity <= 0:
            raise ValueError("quantity must be positive")
Choose one answer
Answer and explanation

Correct answer

B. That the rule behaves correctly at the boundary cases the lesson is trying to make explicit.

Why

Good tests make the design safer to change because they lock in the meaningful behavior rather than surface-level implementation details.

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