Server error: user not defined before used

Question

I'm calling this view (in Django) with an ajax request containing a username to query the database for. It returns a server error that the user object (in the except condition) is being used before it is defined, but it is defined in the line above!

So I thought maybe it was a scope problem, and put user = None at the start of the elif, knowing it would be changed in the try, but then I get None has no value DoesNotExist.

def signup(request):
    if request.method == 'GET':
        return render_to_response('signup.html', context_instance=RequestContext(request))
    elif request.is_ajax(): 
    #query db for user with username provided via ajax, return if it exists
        try:    
            user = User.objects.get(username=request.POST["username"]) 
        except User.DoesNotExist:
            return HttpResponse("false",context_instance=RequestContext(request))
        else:
            return HttpResponse("true", context_instance=RequestContext(request))

function usernameCheck(){
    $.ajax({
        type:'POST',
        url:"http://omnicloud.me/signup", 
      data:{username: $("#username").value}, 
      success:function(response){
        if(response=="true"){
           $('#passAlert').innerHTML("Sorry, that username is already taken")
        }
        },
        headers:{"X-CSRFToken", getCookie('csrftoken')}
    });

    return !($('#passAlert').value == "Sorry, that username is already taken")
}

Answer 1

except user.DoesNotExist: should reference the user model (User) not the user object (user) as in except User.DoesNotExist: