Python网络编程之协程
What is the association?
与子例程一样,协程也是一种程序组件。 相对子例程而言,协程更为一般和灵活,但在实践中使用没有子例程那样广泛。 协程源自Simula和Modula-2语言,但也有其他语言支持。 协程更适合于用来实现彼此熟悉的程序组件,如合作式多任务,迭代器,无限列表和管道。
协程拥有自己的寄存器上下文和栈,协程调度切换时,将寄存器上下文和栈保存到其他地方,在切回来的时候,恢复先前保存的寄存器上下文和栈。因此:协程能保留上一次调用时的状态(即所有局部状态的一个特定组合),每次过程重入时,就相当于进入上一次调用的状态,换种说法:进入上一次离开时所处逻辑流的位置。
协程的优缺点:
优点
无需线程上下文切换的开销
无需原子操作锁定及同步的开销(更改一个变量)
方便切换控制流,简化编程模型
高并发+高扩展性+低成本:一个CPU支持上万的协程都不是问题。所以很适合用于高并发处理。
缺点:
无法利用多核资源:协程的本质是个单线程,它不能多核,协程需要和进程配合才能运行在多CPU上,当然我们日常所编写的绝大部分应用都没有这个必要,除非是CPU密集型应用。
进行阻塞(Blocking)操作(如IO时)会阻塞掉整个程序
实现协程实例
yield
def consumer(name):
print("--->starting eating baozi...")
while True:
new_baozi = yield # 直接返回
print("[%s] is eating baozi %s" % (name, new_baozi))
def producer():
r = con.__next__()
r = con2.__next__()
n = 0
while n < 5:
n += 1
con.send(n) # 唤醒生成器的同时传入一个参数
con2.send(n)
print("\033[32;1m[producer]\033[0m is making baozi %s" % n)
if __name__ == '__main__':
con = consumer("c1")
con2 = consumer("c2")
p = producer()
Greenlet
安装greenlet
pip3 install greenlet
# -*- coding:utf-8 -*-
from greenlet import greenlet
def func1():
print(12)
gr2.switch()
print(34)
gr2.switch()
def func2():
print(56)
gr1.switch()
print(78)
# 创建两个携程
gr1 = greenlet(func1)
gr2 = greenlet(func2)
gr1.switch() # 手动切换
Gevent
Gevent可以实现并发同步或异步编程,在gevent中用到的主要模式是Greenlet, 它是以C扩展模块形式接入Python的轻量级协程,Greenlet全部运行在主程序操作系统进程的内部,但它们被协作式地调度。
安装Gevent
pip3 install gevent
import gevent
def foo():
print('Running in foo')
gevent.sleep(2)
print('Explicit context switch to foo again')
def bar():
print('Explicit context to bar')
gevent.sleep(3)
print('Implicit context switch back to bar')
# 自动切换
gevent.joinall([
gevent.spawn(foo), # 启动一个协程
gevent.spawn(bar),
])
页面抓取
from urllib import request
from gevent import monkey
import gevent
import time
monkey.patch_all() # 当前程序中只要设置到IO操作的都做上标记
def wget(url):
print('GET: %s' % url)
resp = request.urlopen(url)
data = resp.read()
print('%d bytes received from %s.' % (len(data), url))
urls = [
'https://www.python.org/',
'https://www.python.org/',
'https://github.com/',
'https://yw666.blog.51cto.com/',
]
# 串行抓取
start_time = time.time()
for n in urls:
wget(n)
print("串行抓取使用时间:", time.time() - start_time)
# 并行抓取
ctrip_time = time.time()
gevent.joinall([
gevent.spawn(wget, 'https://www.python.org/'),
gevent.spawn(wget, 'https://www.python.org/'),
gevent.spawn(wget, 'https://github.com/'),
gevent.spawn(wget, 'https://yw666.blog.51cto.com/'),
])
print("并行抓取使用时间:", time.time() - ctrip_time)
输出
C:\Python\Python35\python.exe E:/MyCodeProjects/协程/s4.py
GET: https://www.python.org/
47424 bytes received from https://www.python.org/.
GET: https://www.python.org/
47424 bytes received from https://www.python.org/.
GET: https://github.com/
25735 bytes received from https://github.com/.
GET: https://blog.ansheng.me/
82693 bytes received from https://yw666.blog.51cto.com/.
串行抓取使用时间: 15.143015384674072
GET: https://www.python.org/
GET: https://www.python.org/
GET: https://github.com/
GET: https://blog.ansheng.me/
25736 bytes received from https://github.com/.
47424 bytes received from https://www.python.org/.
82693 bytes received from https://yw666.blog.51cto.com/.
47424 bytes received from https://www.python.org/.
并行抓取使用时间: 3.781306266784668
Process finished with exit code 0
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