    def _internal_in_flat_map(self, DataQueueClass):
        '''
        url -> url, image

        input_schema: url
        output_schema: image
        '''
        input_que = DataQueueClass([('url', ColumnType.SCALAR)])
        input_que.put(('http://towhee.io', ))
        input_que.seal()

        output_que = DataQueueClass([('url', ColumnType.SCALAR), ('image', ColumnType.QUEUE)])

        size = 20
        ret = input_que.get()
        url = ret[0]
        output_que.put_dict({'url': url})
        for i in range(size):
            image = 'image' + str(i)
            output_que.put_dict({'image': image})
        output_que.seal()

        self.assertEqual(output_que.size, size)
        i = 0
        while True:
            ret = output_que.get()
            if ret is None:
                break
            self.assertEqual(ret[0], 'http://towhee.io')
            self.assertEqual(ret[1], 'image' + str(i))
            i += 1
url -> url, image

input_schema: url
output_schema: image
tests/unittests/runtime/test_data_queue.py