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: because the function assigns to total, Python treats total as a local variable. The local one has no value yet when += reads it.
The fix: put total = 0 as the first line inside the function.
Remember: keep running totals inside the function. Do not rely on outside variables.
The pattern behind it: scope and closure bugs.
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