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Why is using a mutable default argument dangerous?

Q: Why is using a mutable default argument dangerous?

Default arguments are evaluated **once** at function definition, so a `def f(x, acc=[])` shares the same list across all calls — accumulating unexpectedly. The fix: default to `None` and create a fresh object inside: `acc = acc if acc is not None else []`.

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Explain the mutable default pitfall.

mutable-default

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Default arguments are evaluated once at function definition, so a def f(x, acc=[]) shares the same list across all calls — accumulating unexpectedly. The fix: default to None and create a fresh object inside: acc = acc if acc is not None else [].

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PYThe bug, then the None-sentinel fix

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