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Please, help. I have a data file with 4 columns (userid, movieid, score, timestamp) that looks like this:

196 242 3 881250949
186 302 3 891717742
22 377 1 878887116
196 51 2 880606923
62 257 2 879372434

I am trying to create a nested dictionary that should look like this:

users = '196': [('242', '3'), ('51', '2')], '186': ['302','3'] ...

My code only picks up one tuple (movieid, score) for each userid:

def create_users_dict():
 try:
 users = 
 for line in open('u.data'):
 (id, movieid, rating, timestamp) = line.split('t')[0:4]
 users[id] = (movieid, rating)
 except IOError as ioerr:
 print('There is an error with the file:' + str(ioerr))
 return users
users = create_users_dict()

users = '196': ('51', '2'), '186': ('302', '3')...

Use setdefault:

def create_users_dict():
 try:
 users = 
 for line in open('u.data'):
 uid, movie_id, rating, timestamp = line.split()
 users.setdefault(uid, ).append((movie_id, rating))
 return users
 except IOError as ioerr:
 print('There is an error with the file:' + str(ioerr))
users = create_users_dict()

print(users)

Output

'196': [('242', '3'), ('51', '2')], '62': [('257', '2')], '186': [('302', '3')], '22': [('377', '1')]

A possible alternative is to check if the key (uid) is in the dictionary, in case is missing initialize the value with the empty list and then simply append.

def create_users_dict():
 try:
 users = 
 for line in open('u.dat'):
 uid, movie_id, rating, timestamp = line.split()
 if uid not in users:
 users[uid] = 
 users[uid].append((movie_id, rating))
 return users
 except IOError as ioerr:
 print('There is an error with the file:' + str(ioerr))

As a side note you should not use id as a name because it shadows the built-in function id.