def test_logStderr(self):
        """
        When the _errFlag is set to L{StandardErrorBehavior.LOG},
        L{endpoints._WrapIProtocol} logs stderr (in childDataReceived).
        """
        d = self.ep.connect(self.factory)
        self.successResultOf(d)
        wpp = self.reactor.processProtocol
        log.addObserver(self._stdLog)
        self.addCleanup(log.removeObserver, self._stdLog)

        wpp.childDataReceived(2, b'stderr1')
        self.assertEqual(self.eventLog['executable'], wpp.executable)
        self.assertEqual(self.eventLog['data'], b'stderr1')
        self.assertEqual(self.eventLog['protocol'], wpp.protocol)
        self.assertIn(
            'wrote stderr unhandled by',
            log.textFromEventDict(self.eventLog))

        When the _errFlag is set to L{StandardErrorBehavior.LOG},
        L{endpoints._WrapIProtocol} logs stderr (in childDataReceived).
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_endpoints.py