I created a list of lists:
>>> xs = [[1] * 4] * 3>>> print(xs)[[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]Then, I changed one of the innermost values:
>>> xs[0][0] = 5>>> print(xs)[[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]Why did every first element of each sublist change to 5?
See also:
How do I clone a list so that it doesn't change unexpectedly after assignment? for workarounds for the problem
List of dictionary stores only last appended value in every iteration for an analogous problem with a list of dicts
How do I initialize a dictionary of empty lists in Python? for an analogous problem with a dict of lists