Question

What is the difference between these two declarations of a list of lists?

>>> l = [[]]*4
>>> l
[[], [], [], []]
>>> l[1].append(1)
>>> l
[[1], [1], [1], [1]]

>>> m = [[],[],[],[]]
>>> m[1].append(1)
>>> m
[[], [1], [], []]

Answer 1

The first one creates four references to a single list — it is the same list repeated four times. The second creates four distinct lists. In the first case, when you append to one list, it affects all of them, because they are all the same object. In the second case, each list is a distinct object, so appending to one doesn’t affect the others.