异步编程

1. 异步基础概念 https://ruanyifeng.com/blog/2019/11/python-asyncio.html
2. yield & yield from https://zhuanlan.zhihu.com/p/75347080
3. yield & yield from https://www.jianshu.com/p/b036e6e97c18
4. 异步入门 https://zhuanlan.zhihu.com/p/25228075
5. asyncio 模块 https://mp.weixin.qq.com/s/JyhBVZPeKM5E44jrrA3LEA
6. asyncio 模块 https://zhuanlan.zhihu.com/p/59621713
7. async | await 关键字 https://zhuanlan.zhihu.com/p/27258289

• 回调函数示例 ```python import socket import time

from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ

selector = DefaultSelector() stopped = False urls = {‘/‘, ‘/1’, ‘/2’, ‘/3’, ‘/4’}

class Crawler: def init(self, url, host): self.url = url self.host = host self.response = b’’ self.request = “GET {} HTTP/1.0\r\nHost: {}\r\n\r\n”.format(self.url, self.host)

def fetch(self):
    self.sock = socket.socket()
    self.sock.setblocking(False)
    try:
        self.sock.connect((self.host, 80))
    except BlockingIOError:
        pass
    selector.register(self.sock.fileno(), EVENT_WRITE, self.on_connected)
def on_connected(self, key, mask):
    selector.unregister(key.fd)
    print(f"Connected: {self.request.strip()}")
    self.sock.send(self.request.encode('ascii'))
    selector.register(key.fd, EVENT_READ, self.read_response)
def read_response(self, key, mask):
    global stopped
    chunk = self.sock.recv(4096)
    while chunk:
        self.response += chunk
        chunk = self.sock.recv(4096)
    print(f"Finished: {self.request.strip()}")
    selector.unregister(key.fd)
    urls.remove(self.url)
    if not urls:
        stopped = True

def loop(): while not stopped: events = selector.select() for event_key, event_mask in events: callback = event_key.data callback(event_key, event_mask)

if name == ‘main‘: start = time.time() for url in urls: crawler = Crawler(url, “baidu.com”) crawler.fetch() loop() print(time.time() - start)

> 可以在 `urls` 中只放一个元素，然后断点调试，观察程序流程
- 生成器（yield）示例
```python
import socket
import time
from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ
selector = DefaultSelector()
urls = {'/', '/1', '/2', '/3', '/4'}
stopped = False
class Future:
    def __init__(self):
        self.result = None
        self._callbacks = []
    def add_done_callback(self, fn):
        self._callbacks.append(fn)
    def set_result(self, result):
        self.result = result
        for fn in self._callbacks:
            fn(self)
class Crawler:
    def __init__(self, url, host):
        self.url = url
        self.host = host
        self.request = "GET {} HTTP/1.0\r\nHost: {}\r\n\r\n".format(self.url, self.host)
        self.response = b''
    def fetch(self):
        sock = socket.socket()
        sock.setblocking(False)
        try:
            sock.connect((self.host, 80))
        except BlockingIOError:
            pass
        f = Future()
        def on_connected():
            f.set_result(None)
        selector.register(sock.fileno(), EVENT_WRITE, on_connected)
        yield f
        selector.unregister(sock.fileno())
        print(f"Connected: {self.request.strip()}")
        sock.send(self.request.encode('ascii'))
        global stopped
        while True:
            f = Future()
            def on_readable():
                f.set_result(sock.recv(4096))
            selector.register(sock.fileno(), EVENT_READ, on_readable)
            chunk = yield f
            selector.unregister(sock.fileno())
            if chunk:
                self.response += chunk
            else:
                print(f"Finished: {self.request.strip()}")
                urls.remove(self.url)
                if not urls:
                    stopped = True
                break
class Task:
    def __init__(self, coro):
        self.coro = coro
        f = Future()
        self.step(f)
    def step(self, future):
        try:
            next_future = self.coro.send(future.result)
        except StopIteration:
            return
        next_future.add_done_callback(self.step)
def loop():
    while not stopped:
        events = selector.select()
        for key, mask in events:
            callback = key.data
            callback()
if __name__ == '__main__':
    start = time.time()
    for url in urls:
        crawler = Crawler(url, "baidu.com")
        Task(crawler.fetch())
    loop()
    print(time.time() - start)