def addlist(list):
    """
    Add lists of lists, recursively
    the result is global
    """
    global add
    
    for item in list:
        if type(item) is list: # If item type is list
            addlist(item)
        else:
            add += item

add = 0
addlist([[1, 2], [3, 4, 5], 6])

print add # 21