<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def invoke(self):
 """ Call the external handler to be invoked. """

        # flush to ensure external process can see flags as they currently
        # are, and write flags (flush releases lock)
        unitdata.kv().flush()
        subprocess.check_call([self._filepath, '--invoke', self._test_output], env=os.environ)