Shared row reference bug

Question

Accepted Answer

[[0] * cols] * rows creates rows references to the same inner list object. Mutating one row mutates all of them, so bad shows 99 in every row. The comprehension [[0] * cols for _ in range(rows)] creates a fresh inner list on each iteration, so only row 0 is affected in good. This is one of the most common Python 2D-array bugs in interviews.