Question

I came across a piece of python list decclaration. I am bit confused about the behavior of it.
Can somone please explain this. Appreciate your help.

    >>> v  = [[0]*2]*2
    >>> v
    [[0, 0], [0, 0]]
    >>> v[1][1] = 23
    >>> v
    [[0, 23], [0, 23]]
    >>> v[1][1] = 44
    >>> v
    [[0, 44], [0, 44]]
    >>>

Answer 1

The * operator for lists repeats their contents, as you can clearly see in the output.

However, it does not copy elements, it just copies object references. So in this case, both [0,0 ]s have the same underlying list object, which should explain the phenomenon.

To verify this, try v[0] = [0,44] to assign a new (and thus independent!) list object to the first element of the master list; then re-try changing v[1][1]. This time only one entry will change in the output.