'Create the Flow object.
The Flow is calculated lazily since we don\'t know where this app is
running until it receives a request, at which point redirect_uri can be
calculated and then the Flow object can be constructed.
Args:
request_handler: webapp.RequestHandler, the request handler.'

def _create_flow(self, request_handler):

    if (self.flow is None):
        redirect_uri = request_handler.request.relative_url(self._callback_path)
        self.flow = client.OAuth2WebServerFlow(self._client_id, self._client_secret, self._scope, redirect_uri=redirect_uri, user_agent=self._user_agent, auth_uri=self._auth_uri, token_uri=self._token_uri, revoke_uri=self._revoke_uri, **self._kwargs)
