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: Multiplying a list containing a list ([[0] * cols] * rows) duplicates the reference to the same row object.
The fix: Use a list comprehension to create independent inner lists: [[0] * cols for _ in range(rows)]
Remember: In Python, [list] * n creates n references to the same list in memory.
The pattern behind it: scope and closure bugs.
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<iframe src="https://trybughunt.com/embed/9957c4a3-e128-4ba5-a08b-f889eb23d84f" title="Debugging challenge: Multiplying row list in matrix creates identical row references" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe>
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