Python Internals · interview question

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

Multiple choiceMediumchapter-19-curatedpremiumpython-internalsmutability-traps

python

def add_flag(flags: list[str] | None = None) -> list[str]:
    flags = [] if flags is None else flags
    flags.append("seen")
    return flags

Keep mutability traps 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 mutability traps.Delay clarity until production traffic reveals the edge case and then patch it under pressure.That the rule behaves correctly at the boundary cases the lesson is trying to make explicit.

Answer and explanation

Correct answer

D. 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 mutability traps, what should the test prove first?

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