Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the logic errors pattern
The wrong operator, an inverted condition, a formula the right way round but backwards.
What went wrong: dict.fromkeys() assigns the exact same mutable list instance to every key. Appending to one key mutates all keys.
The fix: Use a dictionary comprehension to allocate an independent list per key: {role: [] for role in roles}
Remember: In Python, dict.fromkeys(keys, []) shares one list across all keys; use a comprehension for independent mutables.
The pattern behind it: logic errors.
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