Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the mutation and copying bugs pattern
Assignment does not copy, a shallow copy still shares its contents, and mutating methods return None.
What went wrong: sum(data) adds the keys of the dict, which are the text "a" and "b", not the values.
The fix: return sum(data.values())
Remember: a dict on its own means its keys. Use .values() or .items() for the rest.
The pattern behind it: mutation and copying bugs.
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