深入理解Python异步编程（上）

本文代码整理自：深入理解Python异步编程（上）

参考：A Web Crawler With asyncio Coroutines

一、同步阻塞方式

import socket

def blocking_way():
    sock = socket.socket()
    # blocking
    sock.connect(('example.com', 80))
    request = 'GET / HTTP/1.0\r\nHost: example.com\r\n\r\n'
    sock.send(request.encode('ascii'))
    response = b''
    chunk = sock.recv(4096)
    while chunk:
        response += chunk
        # blocking
        chunk = sock.recv(4096)
    return response

def sync_way():
    res = []
    for i in range(10):
        res.append(blocking_way())
    return len(res)

def main():
    start = time.time()
    print(sync_way())
    print(time.time() - start)

if __name__ == '__main__':
    import time
    main()

# 5.15s

二、同步多线程方式

import socket
from concurrent import futures

def blocking_way():
    sock = socket.socket()
    # blocking
    sock.connect(('example.com', 80))
    request = 'GET / HTTP/1.0\r\nHost: example.com\r\n\r\n'
    sock.send(request.encode('ascii'))
    response = b''
    chunk = sock.recv(4096)
    while chunk:
        response += chunk
        # blocking
        chunk = sock.recv(4096)
    return response

def thread_way():
    workers = 10
    with futures.ThreadPoolExecutor(workers) as executor:
        futs = {executor.submit(blocking_way) for i in range(10)}
    return len([fut.result() for fut in futs])

def main():
    start = time.time()
    print(thread_way())
    print(time.time() - start)

if __name__ == '__main__':
    import time
    main()

# 0.52s

　　

小提示

Python中的多线程因为GIL的存在，它们并不能利用CPU多核优势，
一个Python进程中，只允许有一个线程处于运行状态。

那为什么结果还是如预期，耗时缩减到了十分之一？

因为在做阻塞的系统调用时，例如sock.connect(),sock.recv()时，当前线程会释放GIL，
让别的线程有执行机会。但是单个线程内，在阻塞调用上还是阻塞的

Python中 time.sleep 是阻塞的，都知道使用它要谨慎，
但在多线程编程中，time.sleep 并不会阻塞其他线程。

　　

三、非阻塞+回调（即异步非阻塞）方式

事件循环+回调     实现单线程内异步编程

事件监听

OS将I/O状态的变化都封装成了事件，如可读事件、可写事件。
并且提供了专门的系统模块让应用程序可以接收事件通知。这个模块就是select。
让应用程序可以通过select注册文件描述符和回调函数。
当文件描述符的状态发生变化时，select 就调用事先注册的回调函数。

select因其算法效率比较低，后来改进成了poll；
再后来又有进一步改进，BSD内核改进成了kqueue模块，而Linux内核改进成了epoll模块。这四个模块的作用都相同，暴露给程序员使用的API也几乎一致，
区别在于kqueue 和 epoll 在处理大量文件描述符时效率更高。

selectors模块

Python标准库提供的selectors模块是对底层select/poll/epoll/kqueue的封装。
DefaultSelector类会根据 OS 环境自动选择最佳的模块，
那在 Linux 2.5.44 及更新的版本上都是epoll了。

　　

#!/usr/bin/python3.5
# encoding: utf-8

import socket
from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ

selector = DefaultSelector()
stopped = False
urls_todo = {'/', '/1', '/2', '/3', '/4', '/5', '/6', '/7', '/8', '/9'}

class Crawler:
    def __init__(self, url):
        self.url = url
        self.sock = None
        self.response = b''

    def fetch(self):
        self.sock = socket.socket()
        self.sock.setblocking(False)
        try:
            self.sock.connect(('example.com', 80))
        except BlockingIOError:
            pass
        selector.register(self.sock.fileno(), EVENT_WRITE, self.connected)

    def connected(self, key, mask):
        selector.unregister(key.fd)
        get = 'GET {0} HTTP/1.0\r\nHost: example.com\r\n\r\n'.format(self.url)
        self.sock.send(get.encode('ascii'))
        selector.register(key.fd, EVENT_READ, self.read_response)

    def read_response(self, key, mask):
        global stopped
        # 如果响应大于4KB，下一次循环会继续读
        chunk = self.sock.recv(4096)
        if chunk:
            self.response += chunk
        else:
            selector.unregister(key.fd)
            urls_todo.remove(self.url)
            if not urls_todo:
                stopped = True

# 事件循环
def loop():
    while not stopped:
        # 阻塞, 直到一个事件发生
        events = selector.select()
        for event_key, event_mask in events:
            callback = event_key.data
            callback(event_key, event_mask)

if __name__ == '__main__':
    import time
    start = time.time()
    for url in urls_todo:
        crawler = Crawler(url)
        crawler.fetch()
    loop()
    print(time.time() - start)

# 0.53s

回调层次过多的缺点：

- 共享状态管理困难

在回调的版本中，我们必须在Crawler实例化后的对象self里保存它自己的sock对象。

如果不是采用OOP的编程风格，那需要把要共享的状态接力似的传递给每一个回调。

多个异步调用之间，到底要共享哪些状态，事先就得考虑清楚，精心设计。

- 错误处理困难

一连串的回调构成一个完整的调用链；
如果其中一环抛了异常怎么办？
整个调用链断掉，接力传递的状态也会丢失，这种现象称为调用栈撕裂。

所以，为了防止栈撕裂，异常必须以数据的形式返回，而不是直接抛出异常，
然后每个回调中需要检查上次调用的返回值，以防错误吞没。

四、Python 对异步I/O的优化之路

#!/usr/bin/python3.5
# encoding: utf-8

import socket
from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ

selector = DefaultSelector()
stopped = False
urls_todo = {'/', '/1', '/2', '/3', '/4', '/5', '/6', '/7', '/8', '/9'}

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):
        self.url = url
        self.response = b''

    def fetch(self):
        sock = socket.socket()
        sock.setblocking(False)
        try:
            sock.connect(('example.com', 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())
        get = 'GET {0} HTTP/1.0\r\nHost: example.com\r\n\r\n'.format(self.url)
        sock.send(get.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:
                urls_todo.remove(self.url)
                if not urls_todo:
                    stopped = True
                break

class Task:
    def __init__(self, coro):
        self.coro = coro
        f = Future()
        f.set_result(None)
        self.step(f)

    def step(self, future):
        try:
            # send会进入到coro执行, 即fetch, 直到下次yield
            # next_future 为yield返回的对象
            next_future = self.coro.send(future.result)
        except StopIteration:
            return
        next_future.add_done_callback(self.step)

# 事件循环
def loop():
    while not stopped:
        # 阻塞, 直到一个事件发生
        events = selector.select()
        for event_key, event_mask in events:
            callback = event_key.data
            callback()

if __name__ == '__main__':
    import time
    start = time.time()
    for url in urls_todo:
        crawler = Crawler(url)
        Task(crawler.fetch())
    loop()
    print(time.time() - start)

# 0.53s

在前辈的基础上做了一点更改：

#!/usr/bin/python3
# encoding: utf-8

import socket
from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ

selector = DefaultSelector()
stopped = False
urls_todo = {'/', '/1', '/2', '/3', '/4', '/5', '/6', '/7', '/8', '/9'}

class Future:
    def __init__(self):
        self.result = None
        self._callback = None   # 原来是用列表来保存

    def add_done_callback(self, fn):
        self._callback = fn

    def set_result(self, result):
        self.result = result
        # 因为只有一个对应的 Task.step()函数
        if self._callback:
            self._callback(self)

class Crawler:
    def __init__(self, url):
        self.url = url
        self.response = b''

    def fetch(self):
        sock = socket.socket()
        sock.setblocking(False)
        try:
            sock.connect(('example.com', 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())
        get = 'GET {0} HTTP/1.0\r\nHost: example.com\r\n\r\n'.format(self.url)
        sock.send(get.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:
                urls_todo.remove(self.url)
                if not urls_todo:
                    stopped = True
                break

class Task:
    def __init__(self, coro):
        self.coro = coro
        f = Future()
        f.set_result(None)
        self.step(f)

    def step(self, future):
        try:
            # send会进入到coro执行, 即fetch, 直到下次yield
            # next_future 为yield返回的对象
            next_future = self.coro.send(future.result)
        except StopIteration:
            return
        next_future.add_done_callback(self.step)
        print(next_future._callback)

# 事件循环
def loop():
    while not stopped:
        # 阻塞, 直到一个事件发生
        events = selector.select()
        for event_key, event_mask in events:
            callback = event_key.data
            callback()

if __name__ == '__main__':
    import time
    start = time.time()
    c_list = []
    for url in urls_todo:
        crawler = Crawler(url)
        Task(crawler.fetch())
        c_list.append(crawler)

    loop()
    # 增加了对爬取内容的输出
    for crawler in c_list:
        print(crawler.response)
    print(time.time() - start)

　　

五、用 yield from 改进生成器协程

yield可以直接作用于普通Python对象，而yield from却不行，

所以我们对Future还要进一步改造，把它变成一个iterable对象就可以了

#!/usr/bin/python3.5
# -*- coding:utf-8 -*-

import socket
from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ

selector = DefaultSelector()
stopped = False
urls_todo = {'/', '/1', '/2', '/3', '/4', '/5', '/6', '/7', '/8', '/9'}

def connect(sock, address):
    f = Future()
    sock.setblocking(False)
    try:
        sock.connect(address)
    except BlockingIOError:
        pass

    def on_connected():
        f.set_result(None)

    selector.register(sock.fileno(), EVENT_WRITE, on_connected)
    yield from f
    selector.unregister(sock.fileno())

def read(sock):
    f = Future()

    def on_readable():
        f.set_result(sock.recv(4096))

    selector.register(sock.fileno(), EVENT_READ, on_readable)
    chunk = yield from f
    selector.unregister(sock.fileno())
    return chunk

def read_all(sock):
    response = []
    chunk = yield from read(sock)
    while chunk:
        response.append(chunk)
        chunk = yield from read(sock)
    return b''.join(response)

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)

    def __iter__(self):
        yield self
        return self.result

class Crawler:
    def __init__(self, url):
        self.url = url
        self.response = b''

    def fetch(self):
        global stopped
        sock = socket.socket()
        yield from connect(sock, ('example.com', 80))
        get = 'GET {0} HTTP/1.0\r\nHost: example.com\r\n\r\n'.format(self.url)
        sock.send(get.encode('ascii'))
        self.response = yield from read_all(sock)
        urls_todo.remove(self.url)
        if not urls_todo:
            stopped = True

class Task:
    def __init__(self, coro):
        self.coro = coro
        f = Future()
        f.set_result(None)
        self.step(f)

    def step(self, future):
        try:
            # send会进入到coro执行, 即fetch, 直到下次yield
            # next_future 为yield返回的对象
            next_future = self.coro.send(future.result)
        except StopIteration:
            return
        next_future.add_done_callback(self.step)

# 事件循环
def loop():
    while not stopped:
        # 阻塞, 直到一个事件发生
        events = selector.select()
        for event_key, event_mask in events:
            callback = event_key.data
            callback()

if __name__ == '__main__':
    import time
    start = time.time()
    for url in urls_todo:
        crawler = Crawler(url)
        Task(crawler.fetch())
    loop()
    print(time.time() - start)

# 0.53s

　　

六、asyncio和原生协程初体验

asyncio是Python 3.4 试验性引入的异步I/O框架（PEP 3156），提供了基于协程做异步I/O编写单线程并发代码的基础设施。

其核心组件有事件循环（Event Loop）、协程(Coroutine）、任务(Task)、未来对象(Future)以及其他一些扩充和辅助性质的模块。

在引入asyncio的时候，还提供了一个装饰器@asyncio.coroutine用于装饰使用了yield from的函数，以标记其为协程。但并不强制使用这个装饰器。

在 3.5 中新增了async/await语法（PEP 492），对协程有了明确而显式的支持，称之为原生协程。

async/await 和 yield from这两种风格的协程底层复用共同的实现，而且相互兼容。

在Python 3.6 中asyncio库“转正”，不再是实验性质的，成为标准库的正式一员。

#!/usr/bin/python3.5
# -*- coding:utf-8 -*-

import asyncio
import aiohttp

host = 'http://example.com'
urls_todo = {'/', '/1', '/2', '/3', '/4', '/5', '/6', '/7', '/8', '/9'}

loop = asyncio.get_event_loop()

async def fetch(url):
    async with aiohttp.ClientSession(loop=loop) as session:
        async with session.get(url) as response:
            response = await response.read()
            return response

if __name__ == '__main__':
    import time
    start = time.time()
    tasks = [fetch(host + url) for url in urls_todo]
    loop.run_until_complete(asyncio.gather(*tasks))
    print(time.time() - start)

# 0.54s

2019-06-26补充demo示例

 import time
 import asyncio
 import requests

 urls = [
     'http://httpbin.org/get',
     'http://httpbin.org/ip',
     'http://httpbin.org/json',
     'http://httpbin.org/uuid',
     'http://httpbin.org/user-agent',
     'http://httpbin.org/headers',
     'http://httpbin.org/response-headers',
 ]

 def get_result(url):
     d = requests.get(url)
     dd = d.json()
     return dd

 start = time.time()

 results = []
 for url in urls:
     d = get_result(url)
     results.append(d)

 print('RUN : {}'.format(time.time()-start))
 print(results)

耗时：RUN : 6.703306198120117

 import time
 import asyncio
 import requests

 urls = [
     'http://httpbin.org/get',
     'http://httpbin.org/ip',
     'http://httpbin.org/json',
     'http://httpbin.org/uuid',
     'http://httpbin.org/user-agent',
     'http://httpbin.org/headers',
     'http://httpbin.org/response-headers',
 ]

 def myfunc(url):
     d = requests.get(url)
     dd = d.json()
     return dd

 @asyncio.coroutine
 def fetch_async(func, url):
     loop = asyncio.get_event_loop()
     future = loop.run_in_executor(None, func, url)
     data = yield from future
     return data

 start = time.time()
 loop = asyncio.get_event_loop()
 tasks = [fetch_async(myfunc, url) for url in urls]
 results = loop.run_until_complete(asyncio.gather(*tasks))
 loop.close()

 print('RUN : {}'.format(time.time()-start))
 print(results)

耗时：RUN : 1.0276665687561035

补充说明：

run_in_executor(self, executor, func, *args) 第一个参数是传入一个executor（即concurrent.futures.ThreadPoolExecutor，线程池对象）,
不传的话，默认使用 (os.cpu_count() or 1) * 5 这个数值，即如果是4核的cpu，就会对应生成一个含有20线程的线程池，来执行传入的第二个函数func.
所以run_in_executor其实开启了新的线程，再协调各个线程。