Constructing a two-dimensional list [duplicate]

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Unexpected feature in a Python list of lists

I tried to create a list in python using following statement

S = [0] * 10

And it worked out well

S[0] = 1
S
[1, 0, 0, 0, 0, 0, 0, 0, 0, 0]

But when I try to generate a two-dimensional list, something unexpected occurs

S = [[0] * 10] * 2
S[0][1] = 1
S[0][2] = 2
S
[[0, 1, 2, 0, 0, 0, 0, 0, 0, 0], [0, 1, 2, 0, 0, 0, 0, 0, 0, 0]]

Note that S[0] == S[1]

Why?

By the way, is this the best approach of constructing an 2d array? If not, what makes the best?

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Because you told it to make 2 copies of the same mutable list.

S = [[0] * 10 for x in range(2)]

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S = [[0] * 10] * 2 means 2 references for [[0]*10], so when your change one the other will change.By the way ,the reason why [0]*10 is fine is : a reference to a number.