[Fixed]-Django: How to check if username already exists?

You can use exists:

from django.contrib.auth.models import User

if User.objects.filter(username=self.cleaned_data['username']).exists():
    # Username exists
    ...

You can check if the username exists with the clean_username method and raise ValidationError:

def clean_username(self, username):
    user_model = get_user_model() # your way of getting the User
    try:
        user_model.objects.get(username__iexact=username)
    except user_model.DoesNotExist:
        return username
    raise forms.ValidationError(_("This username has already existed."))

If this case, you can show the error in the signup form and do not need to redirect to another page.

update:

As per @Spacedman pointed out a valid point regarding to race conditions on checking username uniqueness on Form logic against DB level’s, although your chance of getting this is very unlikely, in case you do here are the relevant SO answers that may worth reading:

How to avoid race condition with unique checks in Django

Race conditions in django

Another update

As per OP’s comment, here’s another change can be made for the views:

def register_user(request):
    # be DRY, the form can be reused for both POST and GET
    form = MyRegistrationForm(request.POST or None)

    # check both request is a POST and the form is valid
    # as you don't need to redirect for form errors, remove else block
    # otherwise it's going to redirect even form validation fails
    if request.method == 'POST' and form.is_valid():
        form.save()
        return HttpResponseRedirect('/accounts/register_success')
    # I use render so need not update the RequestContext, Django does it for you
    html = render(request, 'register.html', {'form': form})
    return HttpResponse(html)

Hope this helps.

if you are using django’s built in Usercreationform then simply type in your template:

{{form.errors}}

This will check almost everything like:

1. whether user is created or not
2. passwords are matching or not

You can use unique=True

in models.py

username = models.CharField(max_length=30, blank=True, null=True, unique=True)