Fetch the credentials (and cache them on disk).

def init(self):
        """
        Fetch the credentials (and cache them on disk).
        """

        self.credentials = self._get_credentials()
        self.http = self.credentials.authorize(httplib2.Http())
        self.service = discovery.build(
            'sheets',
            'v4',
            http=self.http,
            discoveryServiceUrl=DISCOVERY_URL,
        )