[FIXED] Using __contains__ on a list of custom class objects

Issue

I have a simple class defined like this:

class User(object):
    def __init__(self, id=None, name=None):
        self.id = id
        self.name = name

    def __contains__(self, item):
        return item == self.id

Using this class, I can do simple checks of single instances of the class like so:

>>> user1 = User(1, 'name_1')
>>> 1 in user1
True
>>> 2 in user1
False

This is working as expected.

How can I check if a value is in a list of User objects though? It always seems to return False.

Example:

from random import randint
from pprint import pprint
users = [User(x, 'name_{}'.format(x)) for x in xrange(5)]
pprint(users, indent=4)

for x in xrange(5):
    i = randint(2,6) 
    if i in users:
        print("User already exists: {}".format(i))
    else:
        print("{} is not in users list. Creating new user with id: {}".format(i, i))
        users.append(User(i, 'new_user{}'.format(i)))
pprint(users, indent=4)

This creates output similar to this:

[   0 => name_0, 
    1 => name_1, 
    2 => name_2, 
    3 => name_3, 
    4 => name_4]
6 is not in users list. Creating new user with id: 6
6 is not in users list. Creating new user with id: 6
6 is not in users list. Creating new user with id: 6
3 is not in users list. Creating new user with id: 3
3 is not in users list. Creating new user with id: 3
[   0 => name_0,
    1 => name_1,
    2 => name_2,
    3 => name_3,
    4 => name_4,
    6 => new_user6,
    6 => new_user6,
    6 => new_user6,
    3 => new_user3,
    3 => new_user3]

The issue is that the user with id 6 should have only been created 1 time, because it wasn't created already. The second and third time that 6 was attempted, it should have failed. The user would id 3 shouldn't have been recreated at all because it was part of the initialization of the users variable.

How do I modify by __contains__ method to be able to properly utilize in when comparing against multiple instances of my class?

Solution

This is a misuse of __contains__. You would expect to implement __contains__ on a class like UserList.

A better way to do this is to just access the id attribute directly in generator expression or list comprehension (rather than using the in operator). eg.

class User(object):
    def __init__(self, id=None, name=None):
        self.id = id
        self.name = name

user = User(1, 'name_1')
assert 1 == user.id

user_list = [user, User(2, 'name_2')]
assert any(2 == u.id for u in user_list)

Then for your random example, you'd use a set or dictionary to store the ids of the users that already exist.

users = [User(x, 'name_{}'.format(x)) for x in xrange(5)]
ids = set(u.id for u in users)

for x in xrange(5):
    i = randint(2,6) 
    if i in ids:
        print("User id already exists: {}".format(i))
    else:
        print("{} is not in users list. Creating new user with id: {}".format(i, i))
        ids.add(i)
        users.append(User(i, 'new_user{}'.format(i)))

Answered By - Dunes