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: defaults.update() changes the caller's own defaults dict. The next caller gets the changed one.
The fix: merged = dict(defaults), then merged.update(overrides), then return merged.
Remember: a function should not change the dicts or lists passed into it unless that is its job.
The pattern behind it: mutation and copying bugs.
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