def test_removeAll(self):
        """
        C{reactor.removeAll()} removes all registered L{IReadDescriptor}
        providers and all registered L{IWriteDescriptor} providers and returns
        them.
        """
        reactor = self.buildReactor()

        reactor, fd, server = self._simpleSetup()

        fd.doRead = lambda: self.fail("doRead should not be called")
        fd.doWrite = lambda: self.fail("doWrite should not be called")

        server.sendall(b'x')

        reactor.addReader(fd)
        reactor.addWriter(fd)

        removed = reactor.removeAll()

        # Give the reactor two timed event passes to notice that there's I/O
        # (if it is incorrectly watching for I/O).
        reactor.callLater(0, reactor.callLater, 0, reactor.stop)

        self.runReactor(reactor)
        # Getting here means the right thing happened probably.

        self.assertEqual(removed, [fd])

        C{reactor.removeAll()} removes all registered L{IReadDescriptor}
        providers and all registered L{IWriteDescriptor} providers and returns
        them.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_fdset.py