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: .copy() makes a new outer dict, but the inner "display" dict is the same object in both. Changing it changes the original.
The fix: import copy and use updated = copy.deepcopy(settings)
Remember: .copy() is shallow. For dicts or lists inside dicts, use copy.deepcopy().
The pattern behind it: mutation and copying bugs.
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