code: def authenticate(self, me, state=None, next_url=None):
          
          redirect_url = flask.url_for(
              self.flask_endpoint_for_function(self._authenticated_handler),
              _external=True)
          return self._start_indieauth(me, redirect_url, state or next_url, None)
docs: Authenticate a user via IndieAuth.
  
          Args:
            me (string): the authing user's URL. if it does not begin with
              https?://, http:// will be prepended.
            state (string, optional): passed through the whole auth process,
              useful if you want to maintain some state, e.g. the starting page
              to return to when auth is complete.
            next_url (string, optional): deprecated and replaced by the more
              general "state". still here for backward compatibility.
  
          Returns:
            a redirect to the user's specified authorization url, or
            https://indieauth.com/auth if none is provided.