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A bug appears around validation. What is the strongest first debugging move?

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
Choose one answer
Answer and explanation

Correct answer

D. Trace one concrete example, inspect the state transitions, and compare them to the intended rule before rewriting anything.

Why

That approach is reliable because it creates evidence. Allowing invalid data deeper into the service layer makes later failures harder to interpret.

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