twit_heathens_2.py

#!/usr/bin/env python
import twitter
import sys
import os
from getpass import getpass

def looping_call(fun):
    page = 1
    while True:
        results_page = fun(page=page)
        for result in results_page:
            yield result
        if len(results_page) < 100:
            break
        page += 1

def call_api(username,password):
    api = twitter.Api(username,password)
    friends = looping_call(api.GetFriends)
    followers = list(looping_call(api.GetFollowers))
    heathens = filter(lambda x: x not in followers,friends)
    print "There are %i people you follow who do not follow you:" % len(heathens)
    for heathen in heathens:
        print heathen.screen_name
        
if __name__ == "__main__":
    password = getpass()
    call_api(sys.argv[1], password)

You could also simply get the difference of the two sets and avoid the lambda function, I think the following code would work:

friends = looping_call(api.GetFriends)
followers = looping_call(api.GetFollowers)
heathens = list(set(friends).difference(set(followers)))