FastAPI · interview question

If you wrote a test around validation, what should the test prove first?

Multiple choiceMediumchapter-13-curatedpremiumfastapivalidation

python

class CreateOrderRequest(BaseModel):
    sku: str
    quantity: int

    @field_validator("quantity")
    @classmethod
    def quantity_must_be_positive(cls, value: int) -> int:
        if value <= 0:
            raise ValueError("quantity must be positive")
        return value

That the rule behaves correctly at the boundary cases the lesson is trying to make explicit.Delay clarity until production traffic reveals the edge case and then patch it under pressure.Keep validation implicit so the script stays short and hope the next engineer infers the rule.Solve the symptom with comments only and avoid changing the code shape around validation.

Answer and explanation

Correct answer

A. 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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If you wrote a test around validation, what should the test prove first?

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