Python Internals · interview question

What is the healthiest performance judgment around mutability traps?

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.Measure before optimizing, then change the part that actually dominates cost instead of guessing from style alone.

Answer and explanation

Correct answer

D. Measure before optimizing, then change the part that actually dominates cost instead of guessing from style alone.

Why

Premature optimization is usually weaker than targeted optimization backed by measurement.

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What is the healthiest performance judgment around mutability traps?

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