Sends a command to the device.

    Args:
      command: The command to send.
      arg: Optional argument to the command.

def send_command(self, command, arg=None):
    """Sends a command to the device.

    Args:
      command: The command to send.
      arg: Optional argument to the command.
    """
    if arg is not None:
      command = '%s:%s' % (command, arg)
    self._write(six.StringIO(command), len(command))