Checking if username exists in Django

When ModelForms are bound to a model object, they have an attribute called 'instance', which is the model object itself. In your view, when request.method == 'POST', you're probably creating the form instance like this:

form = ChangeNameForm(request.POST, instance=request.user)

If that's the case, you can access the logged user from the form methods, and your validation method can be something like this:

def clean_username(self):
    username = self.cleaned_data['username']
    try:
        user = User.objects.exclude(pk=self.instance.pk).get(username=username)
    except User.DoesNotExist:
        return username
    raise forms.ValidationError(u'Username "%s" is already in use.' % username)

Consider using the .exists method, for it issues a faster query to your database than if you try to retrieve all the user information with the .get method. And the code gets a little cleaner too:

def clean_username(self):
    username = self.cleaned_data['username']
    if User.objects.exclude(pk=self.instance.pk).filter(username=username).exists():
        raise forms.ValidationError(u'Username "%s" is already in use.' % username)
    return username

Optionally, you can also follow these guidelines when raising the ValidationError.

I can't test this code right now, so I apologize if there's anything wrong.

You can write function to check the username if exists like this:

@ggorlen, thanks! Update:

from django.contrib.auth.models import User

def username_exists(username):
    return User.objects.filter(username=username).exists()

---

from django.contrib.auth.models import User

def username_exists(username):
    if User.objects.filter(username=username).exists():
        return True
    
    return False