python 多次元配列 リストではまる

>>> a = [[] for i in xrange(4)]
>>> a
[[], [], [], []]
>>> b = [a for i in xrange(3)]
>>> b
[[[], [], [], []], [[], [], [], []], [[], [], [], []]]
>>> b[0][0]=1
>>> b[2][0]=10
>>> b[1][0]=2
>>> b
[[2, [], [], []], [2, [], [], []], [2, [], [], []]]

なんでこうなるの？

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