python aiohttp异步限制并发量，设置session=None

如下代码中session = None是什么意思？

import asyncio,aiohttp

CONCURRENCY = 5
URL = 'https://www.baidu.com'

semaphore = asyncio.Semaphore(CONCURRENCY)
session = None

async def scrape_api():
    async with semaphore:
        print('Scraping',URL)
        async with session.get(URL) as response:
            await asyncio.sleep(1)
            return await response.text()

async def main():
    global session
    session = aiohttp.ClientSession()
    scrape_index_tasks = [asyncio.ensure_future(scrape_api()) for _ in range(100)]
    await asyncio.gather(*scrape_index_tasks)

if __name__ == '__main__':
    asyncio.get_event_loop().run_until_complete(main())

定义一个全局变量，随便给了一个默认值，可以为None，可以为其他的，因为main里面或修改他的定义，scrape_api再调用