class RedirectURLMixin

def get_default_redirect_url(self):
    """Return the default redirect URL."""
    if self.next_page:
        return resolve_url(self.next_page)
    raise ImproperlyConfigured("No URL to redirect to. Provide a next_page.")

def get_redirect_url(self):
    """Return the user-originating redirect URL if it's safe."""
    redirect_to = self.request.POST.get(
        self.redirect_field_name, self.request.GET.get(self.redirect_field_name)
    )
    url_is_safe = url_has_allowed_host_and_scheme(
        url=redirect_to,
        allowed_hosts=self.get_success_url_allowed_hosts(),
        require_https=self.request.is_secure(),
    )
    return redirect_to if url_is_safe else ""

def get_success_url(self):
    return self.get_redirect_url() or self.get_default_redirect_url()

def get_success_url_allowed_hosts(self):
    return {self.request.get_host(), *self.success_url_allowed_hosts}