def checkin(self, place_id, sensor=False):
        """Checks in a user to a place.

        keyword arguments:
        place_id  -- The unique Google identifier for the relevant place.
        sensor    -- Boolean flag denoting if the location came from a
                     device using its location sensor (default False).
        """
        data = {'placeid': place_id}
        url, checkin_response = _fetch_remote_json(
                GooglePlaces.CHECKIN_API_URL % (str(sensor).lower(),
                        self.api_key), json.dumps(data), use_http_post=True)
        _validate_response(url, checkin_response)