What to do when Django query returns none? It gives me error

Question

to_friend = User.objects.filter(username=friend_q)[0:1]

If 'friend_q' is NOT inside the User.username...it will give error. What is the recommended tactic?

Thank you

Answer 1

If friend_q is not a user present in the database, to_friend will be equal to an empty list.

>>> from django.contrib.auth.models import User
>>> User.objects.filter(username='does-not-exist')
[]

However, it's better to use the get() method to lookup a specific entry:

>>> User.objects.get(username='does-exist')
<User: does-exist>
>>> User.objects.get(username='does-not-exist')
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/lib/python2.5/django/db/models/manager.py", line 120, in get
  File "/usr/lib/python2.5/django/db/models/query.py", line 305, in get
DoesNotExist: User matching query does not exist.

You can now catch the DoesNotExist exception and take appropriate actions.

try:
   to_friend = User.objects.get(username=friend_q)
except User.DoesNotExist:
   # do something, raise error, ...

Answer 2

1. FWIW, Username is unique. So U can use

   to_friend = User.objects.get(username=friend_q)
   

2. If you want to raise a 404 when the user doesn't exist, you may as well,

   from django.shortcuts import get_object_or_404
   to_friend = get_object_or_404(User,username=friend_q)
   

3. To prevent error, you may simply put it in the try except block, very pythonic.

   try:
      to_friend = User.objects.get(username=friend_q)
   except User.DoesNotExist:
      to_friend = None
   

4. Since this is a common requirement, you should consider defining get_user_or_none on the UserManager, and other managers that would require None return, similar to get_object_or_404. This was in consideration for django core, but was not included, for what ever the reason. Still, is a handy, 4 line function, for models that need None back.

Answer 3

The reason you're getting an error is because you're trying to evaluate the Query object when there's nothing in it. The [0:1] at the end is attempting to take the first item out of an empty list. If you split up the expression a little like below, you can check for an empty list before taking an element out of it.

to_friends = User.objects.filter(username=friend_q)
if to_friends:
    to_friend = to_friends[0]