def test_installHandler(self):
        """
        The file descriptor passed to L{installHandler} has a byte written to
        it when SIGCHLD is delivered to the process.
        """
        read, write = self.pipe()
        installHandler(write)

        exc = self.assertRaises(OSError, os.read, read, 1)
        self.assertEqual(exc.errno, errno.EAGAIN)

        os.kill(os.getpid(), signal.SIGCHLD)

        self.assertEqual(len(os.read(read, 5)), 1)

        The file descriptor passed to L{installHandler} has a byte written to
        it when SIGCHLD is delivered to the process.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_sigchld.py