Note that your subject line is wrong. You are not doing list concatenation.
Unfortunately, for technical and optimization reasons, the += assignment operator for lists is in-place, which means that it is NOT the same as ordinary list concatenation + operator. It is equivalent to calling the list.extend method. If you want to do concatenation, creating NEW lists on every operation, you have to write it the expanded way: list1 = list1 + list2 # creates a new list from the concatenation of list1 and list2 Your code starts like this: list1 = list2 = [1, 2, 3] That is NOT two different lists with equal contents, but ONE list with two names. You can see this with this: py> list1 = list2 = [1, 2, 3] py> list1.append(4) py> list2 [1, 2, 3, 4] py> list1 is list2 # Are they the same object? True So what happens when you do an in-place extend of a list to itself? You should be able to guess from the example of append: py> list1.extend(list2) py> list2 [1, 2, 3, 4, 1, 2, 3, 4] list1 will be the same, because list1 and list2 are the same list. Using += will give the same results. So you have two gotchas here: a = b = [1, 2, 3] gives you the same list with two names. You could make a copy: a = [1, 2, 3] b = a[:] # using slicing or: import copy a = [1, 2, 3] b = copy.copy(a) The second gotcha is that += is not always equivalent to + and in particular for lists it is equivalent to calling the extend() method. -- Steve “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough, things got worse. -- https://mail.python.org/mailman/listinfo/python-list
