Exercise 3 solution

    import copy

    bad = [[0] * 3] * 3
    good = [[0] * 3 for _ in range(3)]
    bad[1][1] = 5
    good[1][1] = 5
    print(bad)
    print(good)

Output:
    [[0, 5, 0], [0, 5, 0], [0, 5, 0]]
    [[0, 0, 0], [0, 5, 0], [0, 0, 0]]

Explanation: [[0] * 3] * 3 repeats one reference to a single row three
times, so a change through any row shows in all of them. The comprehension
runs [0] * 3 afresh on every pass, creating three separate rows.

Independent copy (a shallow copy such as good[:] would still share rows):

    clone = copy.deepcopy(good)
    clone[0][0] = 9
    print(good)
    print(clone)

Output:
    [[0, 0, 0], [0, 5, 0], [0, 0, 0]]
    [[9, 0, 0], [0, 5, 0], [0, 0, 0]]