def test_removeAllSkipsInternalReaders(self):
        """
        Any L{IReadDescriptors} in L{PosixReactorBase._internalReaders} are
        left alone by L{PosixReactorBase._removeAll}.
        """
        reactor = TrivialReactor()
        extra = object()
        reactor._internalReaders.add(extra)
        reactor.addReader(extra)
        reactor._removeAll(reactor._readers, reactor._writers)
        self._checkWaker(reactor)
        self.assertIn(extra, reactor._internalReaders)
        self.assertIn(extra, reactor._readers)

        Any L{IReadDescriptors} in L{PosixReactorBase._internalReaders} are
        left alone by L{PosixReactorBase._removeAll}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_posixbase.py