I read Python list + list vs. list.append(), which is a similar question, but my question is more in relation to the code below
a = [[]] * 4 b = [[]] * 4 a[3] = a[3] + [1] b[3].append(1) print a, b Which gives:
[[],[],[],[1]] [[1],[1],[1],[1]] Why would these 2 be any different? I've never run into an example like this where these 2 methods have different outputs...
Thanks
a[3] = a[3] + [1] is not modifying a[3]. Instead, it is putting a new item there. a[3] + [1] creates a list that is just like a[3] except that it has an extra one at the end. Then, a[3] = ... sets a at the index 3 to that new list.
b[3].append(1) accesses b[3] and uses its .append() method. The .append() method works on the list itself and puts a one at the end of the list. Since [[]] * 4 creates a list with four copies of another list, the .append() method reveals its changes in all items of b.
