async def anext(async_iterator):
    '''
    获取异步迭代器的下一数据项，
    没有下一项时产生 StopAsyncIteration 异常

    :param async_iterator: 一个异步迭代器
    :return: 迭代器的下一项
    '''

async def anext(async_iterator, default):
    '''
    获取异步迭代器的下一数据项，没有下一项时返回 default

    :param async_iterator: 一个异步迭代器
    :param default: 默认值
    :return: 迭代器的下一项
    '''

import asyncio

# 异步迭代器类
class AsyncIterator:
    def __init__(self, stop):
        self.__stop = stop
        self.__current = 0
    
    async def __anext__(self):
        if self.__current < self.__stop:
            await asyncio.sleep(0.1)  # 模拟异步操作
            self.__current += 1
            return self.__current - 1
        else:
            raise StopAsyncIteration

# 异步函数
async def main():
    # 异步迭代器对象
    async_iterator = AsyncIterator(10)

    # 迭代
    while (value := await anext(async_iterator, None)) is not None:
        print(value)

# 启动
asyncio.run(main())