index = { WORD, [url1,url2,url3] }

def update_index(word, url):
    if word in index and not(url in index[word]):
       index[word].append(url) # list already exists append to it
    else:
       index[word] = [url] # new list with url as a single element

def update_index(word, url):
    if word in index:                  # <- isnt having two consecutive if statements
                                       # the same as an AND???
       if not(url in index[word]):
          index[word].append(url) # list already exists append to it
    else:
       index[word] = [url] # new list with url as a single element

from collections import defaultdict

url_store = defaultdict(set)
url_store[word].add(url)

if word not in index:
    index[word] = [] # create new empty list for word
# now we know that a list exists -> append
if url not in index[word]:
    index[word].append(url)