Why does my list disappear when I exit a while loop?

Question

I have a piece of code that loops until an error is raised or a value of None is assigned to the variable "parent." On each loop, it appends a value to a list, known as actionList. Through print statements, I know that the list is growing through each iteration of the loop. However, after I exit the loop (through a KeyError that is caught), print statements reveal that the list is now assigned a value of None. Any idea what's going on?

while parent is not None:
                print "iterating"
                print actionList
                try:
                    pathList.append(parentMap[parent])
                except KeyError:
                    break
                actionList.append(actionMap[parent])
                parent = parentMap.get(parent, None)
            return actionList.reverse()

Upon further examination, it looks like the actionList is still intact once I leave the loop. However, when the reverse() method is used on actionList, it disappears. Am I using reverse() wrong?

Answer 1

Yes. reverse reverses the list in place and returns None. Do this instead:

actionList.reverse()
return actionList

You could also do return list(reversed(actionList)) , which will return a reversed copy of the list.