python - aiohttp 限制并发请求数

Question

我正在使用 aiohttp 下载图像，想知道是否有办法限制未完成的打开请求的数量。这是我目前拥有的代码:

async def get_images(url, session):

    chunk_size = 100

    # Print statement to show when a request is being made. 
    print(f'Making request to {url}')

    async with session.get(url=url) as r:
        with open('path/name.png', 'wb') as file:
            while True:
                chunk = await r.content.read(chunk_size)
                if not chunk:
                    break
                file.write(chunk)

# List of urls to get images from
urls = [...]

conn = aiohttp.TCPConnector(limit=3)
loop = asyncio.get_event_loop()
session = aiohttp.ClientSession(connector=conn, loop=loop)
loop.run_until_complete(asyncio.gather(*(get_images(url, session=session) for url in urls)))

问题是，当每个请求被发出时，我扔了一个打印语句告诉我，它一次发出近 21 个请求，而不是我想限制的 3 个(即，一次图像下载完成后，它可以移动到列表中的下一个 url 来获取)。我只是想知道我在这里做错了什么。

Answer 1

您的限制设置工作正常。你在调试时犯了错误。

正如 Mikhail Gerasimov 在 the comment 中指出的那样，您将 print() 调用放在错误的位置 - 它必须在 session.get() 上下文中。

为了确信限制得到遵守，我针对简单的日志记录服务器测试了您的代码 - 测试表明服务器收到的连接数与您在 TCPConnector 中设置的连接数完全相同。这是测试:

import asyncio
import aiohttp
loop = asyncio.get_event_loop()


class SilentServer(asyncio.Protocol):
    def connection_made(self, transport):
        # We will know when the connection is actually made:
        print('SERVER |', transport.get_extra_info('peername'))


async def get_images(url, session):

    chunk_size = 100

    # This log doesn't guarantee that we will connect,
    # session.get() will freeze if you reach TCPConnector limit
    print(f'CLIENT | Making request to {url}')

    async with session.get(url=url) as r:
        while True:
            chunk = await r.content.read(chunk_size)
            if not chunk:
                break

urls = [f'http://127.0.0.1:1337/{x}' for x in range(20)]

conn = aiohttp.TCPConnector(limit=3)
session = aiohttp.ClientSession(connector=conn, loop=loop)


async def test():
    await loop.create_server(SilentServer, '127.0.0.1', 1337)
    await asyncio.gather(*(get_images(url, session=session) for url in urls))

loop.run_until_complete(test())