code: def _dev_by_id(self, device_type):
          
          dir = os.path.join("/dev", device_type, "by-id")
          if os.path.isdir(dir):
              to_ret = dict(
                  self._hex_ids([os.path.join(dir, f) for f in os.listdir(dir)])
              )
              return to_ret
          else:
              logger.error(
                  "Could not get %s devices by id. "
                  "This could be because your Linux distribution "
                  "does not use udev, or does not create /dev/%s/by-id "
                  "symlinks. Please submit an issue to github.com/"
                  "armmbed/mbed-ls.",
                  device_type,
                  device_type,
              )
              return {}
docs: ! Get a dict, USBID -> device, for a device class
          @param device_type The type of devices to search. For exmaple, "serial"
            looks for all serial devices connected to this computer
          @return A dict: Device USBID -> device file in /dev