Nested Lists Assignment Problem

[Licheng Fang]

I wanna use nested lists as an array, but here's the problem:

>>> a = [[0]*3]*3
>>> a
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> a[0][0] = 1
>>> a
[[1, 0, 0], [1, 0, 0], [1, 0, 0]]

Could anybody please explain to me why three values were change? I'm
bewildered. Thanks!