Dictionary is not being added to - python-3.x

Question

I'm trying to create a dictionary using keys and values from other dictionaries. The issue I'm having is that the dictionary I want to return comes up empty.
Note:
USER_RATING_DICT_SMALL = {1: {68735: 3.5, 302156: 4.0}, 2: {68735: 1.0, 124057: 1.5, 293660: 4.5}}
MOVIE_USER_DICT_SMALL = {293660: [2], 68735: [1, 2], 302156: [1], 124057: [2]}
def movies_to_users(user_ratings):
"""Return a dictionary of movie ids to list of users who rated the movie,
using information from the user_ratings dictionary of users to movie
ratings dictionaries.
>>> result = movies_to_users(USER_RATING_DICT_SMALL)
>>> result == MOVIE_USER_DICT_SMALL
True
"""
user_list = []
movie_to_users = {}
for items in user_ratings.items():
user_id = items[0]
user_list.append(user_id)
for items in user_ratings.items():
movie_id = items[1]
if movie_id in user_list:
movie_to_users[movie_id] = [user_list]
return movie_to_users
I created an empty dictionary for all the values and keys to accumulate to but it is not accumulating; it returns an empty dictionary instead. I want the output to be == MOVIE_USER_DICT_SMALL

Answer 1

A pretty short solution might be
MOVIE_USER_DICT_SMALL = {}
for user, data in USER_RATING_DICT_SMALL.items():
for movie in data.keys():
if not movie in MOVIE_USER_DICT_SMALL.keys():
# add entry for each movie
# will be an empty list if no user rated.
MOVIE_USER_DICT_SMALL[movie] = []
# for user in current iteration, there MUST be a rating...
# otherwise, movie would not be in data.keys()
MOVIE_USER_DICT_SMALL[movie] += [user]
MOVIE_USER_DICT_SMALL
# {68735: [1, 2], 302156: [1], 124057: [2], 293660: [2]}
The trick is to use key and value as returned by dict.items()