docs: def clean_username(self):
          """
          Validate that the username is alphanumeric and is not already
          in use. Don't fail if users username is provided.
          """
          user = None
          try:
              user = User.objects.get(username__iexact=self.\
                      cleaned_data['username'])
          except User.DoesNotExist:
              return self.cleaned_data['username']
  
          if user:
              if user.username == self.user.username:
                  return self.cleaned_data['username']
  
          raise forms.ValidationError(_(\
                  "A user with that username already exists."))