Returns the firmware version of the sensor if available. Currently only
        I2C/NXT sensors support this.

def fw_version(self):
        """
        Returns the firmware version of the sensor if available. Currently only
        I2C/NXT sensors support this.
        """
        (self._fw_version, value) = self.get_cached_attr_string(self._fw_version, 'fw_version')
        return value