def test_count(self):
        """
        L{_ExhaustsFileDescriptors.count} returns the number of open
        file descriptors.
        """
        self.assertEqual(self.exhauster.count(), 0)
        self.exhauster.exhaust()
        self.assertGreater(self.exhauster.count(), 0)
        self.exhauster.release()
        self.assertEqual(self.exhauster.count(), 0)

        L{_ExhaustsFileDescriptors.count} returns the number of open
        file descriptors.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_tcp.py