What does this function return on the second call?

Question

Accepted Answer

["a", "b"]. Default arguments are evaluated once, when the function is defined — not on each call. Every call that does not pass basket shares the same list object, so mutations accumulate across calls. The fix is the standard idiom: This matters well beyond interviews. The bug is subtle in production because it only shows up on the second call, which is often in a different request — making it look like a state-leak bug in a framework rather than a language semantics issue.