Combining elements in list: seems like python treats the same item in two different ways and I don't know why [duplicate]

Answer 1

Martijn (as always) has explained this well. However, the (for reference only) Pythonic approach would be:

def join_lists(*args):
    from itertools import chain
    return list(chain.from_iterable(args))

Answer 2

The += in-place add operator on a list does the same thing as calling list.extend() on new_list. .extend() takes an iterable and adds each and every element to the list.

list.append() on the other hand, adds a single item to the list.

>>> lst = []
>>> lst.extend([1, 2, 3])
>>> lst
[1, 2, 3]
>>> lst.append([1, 2, 3])
>>> lst
[1, 2, 3, [1, 2, 3]]