>>> L = [[2], [2]] * 5
>>> L
[[2], [2], [2], [2], [2], [2], [2], [2], [2], [2]]
>>> L[3][0] = 9
>>> L
[[2], [9], [2], [9], [2], [9], [2], [9], [2], [9]]
This is because the same two lists are repeated over and over
>>> [id(x) for x in L]
[140053788793352, 140053788793288, 140053788793352, 140053788793288, 140053788793352, 140053788793288, 140053788793352, 140053788793288, 140053788793352, 140053788793288]
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