def test_callablesCalledInOrder(self):
        """
        Check that the callables are called in the given order, one after the
        other.
        """
        log = []
        deferreds = []

        def append(value):
            d = defer.Deferred()
            log.append(value)
            deferreds.append(d)
            return d

        util._runSequentially([lambda: append('foo'),
                               lambda: append('bar')])

        # runSequentially should wait until the Deferred has fired before
        # running the second callable.
        self.assertEqual(log, ['foo'])
        deferreds[-1].callback(None)
        self.assertEqual(log, ['foo', 'bar'])

        Check that the callables are called in the given order, one after the
        other.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_util.py