Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the scope and closure bugs pattern
Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.
What went wrong: basket=[] is created only once. Every call without a basket gets that same list, so it keeps growing.
The fix: def add_item(item, basket=None): then if basket is None: basket = []
Remember: never use a list or dict as a default value. Use None and create it inside.
The pattern behind it: scope and closure bugs.
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