Python Internals · interview question

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

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

Trace one concrete example, inspect the state transitions, and compare them to the intended rule before rewriting anything.Keep mutability traps implicit so the script stays short and hope the next engineer infers the rule.Delay clarity until production traffic reveals the edge case and then patch it under pressure.Solve the symptom with comments only and avoid changing the code shape around mutability traps.

Answer and explanation

Correct answer

A. 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. Mutable defaults and hidden shared containers create bugs that feel spooky until you know the model.

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

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