docs: def clean_username(self):
          """
          Ensure the username doesn't exist or contain invalid chars.
          We limit it to slugifiable chars since it's used as the slug
          for the user's profile view.
          """
          username = self.cleaned_data.get("username")
          if username.lower() != slugify(username).lower():
              raise forms.ValidationError(
                  ugettext("Username can only contain letters, numbers, dashes "
                           "or underscores."))
          lookup = {"username__iexact": username}
          try:
              User.objects.exclude(id=self.instance.id).get(**lookup)
          except User.DoesNotExist:
              return username
          raise forms.ValidationError(
                              ugettext("This username is already registered"))