Python: reference and dereference of global variables in functions

Question

I have a specific problem in which I observe all the confusion of reference and dereference in python. I have a global structure wordhistory which I change on various levels inside a function addWordHistory :

wordhistory = dict()

def addWordHistory(words):
    global wordhistory
    current = wordhistory
    for word in words:
        if current is None:
            current = {word:[None,1]}    #1
        else:
            if word in current:
                current[word][1] += 1
            else:
                current[word] = [None,1]
    current = current[word][0]           #2

In line #1 , I want to change the value behind the reference that has been assigned to the local variable current in line #2 . This does not seem to work like this. Instead, I suspect that only the local variable is changed from a reference to a dictionary.

The below variant works, but I want to save the memory of all the empty leave dictionaries:

wordhistory = dict()

def addWordHistory(words):
    global wordhistory
    current = wordhistory
    for word in words:
        if word in current:
            current[word][1] += 1
        else:
            current[word] = [dict(),1]
        current = current[word][0]

Answer 1

To be able to change an item of the current list, you need to store the reference to the list, not just to the item you need to change:

def addWordHistory(words):
    current = [wordhistory, 0]
    for word in words:
        if current[0] is None:
            current[0] = dict()
        children = current[0]
        if word in children:
            children[word][1] += 1
        else:
            children[word] = [None, 1]
        current = children[word]