why can't I change only a single element in a nested list in Python

Question

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Answer 1

A strange behaviour indeed, but that's only because * operator makes shallow copies, in your case - shallow copies of [0, 0, 0] list. You can use the id() function to make sure that these internal lists are actually the same:

out=[[0]*3]*3
id(out[0])
>>> 140503648365240
id(out[1])
>>> 140503648365240
id(out[2])
>>> 140503648365240

Comprehensions can be used to create different lists as follows:

out = [ [0]*3 for _ in range(3) ]