理解select模块--等待IO完成
用一句话描述select是做什么的: 等待IO完成.
怎么理解
在没有select模块时，通常的socket通信如下
服务器server.py
#encoding=utf-8
import socket
def main():
'''阻塞'''
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(('0.0.0.0', 8080))
sock.listen(5)
while True:
client_sock, address = sock.accept()
client_sock.settimeout(5)
msg = client_sock.recv(1024)
if msg == '1':
client_sock.send('welcome to server!')
else:
client_sock.send('please go out!')
client_sock.close()
if __name__ == '__main__':
main()
客户端client.py
#encoding=utf-8
import socket
import time
def main():
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(('0.0.0.0', 8080))
time.sleep(2)
sock.send('1')
print sock.recv(1024)
sock.close()
if __name__ == '__main__':
main()
启动server.py之后，它会停在socket.accept()上，除了等待，程序再也不能做其它事情了。这样的IO等待，极大影响程序效率。
很自然的会想: 有什么办法把这个等待过程独立出来，不让它影响到整个程序？
select模块就是为了解决这个问题的.
替换上面的server.py
#encoding=utf-8
import socket
def main():
'''非阻塞'''
import select
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(("0.0.0.0", 8080))
sock.listen(5)
while True:
# 传入需要等待的socket，返回准备好的socket
infds, outfds, errfds = select.select([sock, ], [], [], 5)
print infds, outfds, errfds
# 返回准备好的fd
if sock in infds:
client_sock, clientaddr = sock.accept()
msg = client_sock.recv(8196)
if len(msg) != 0:
print (msg)
client_sock.close()
print "no data coming"
if __name__ == '__main__':
main()
上面用法就是将需要等待IO的socket作为参数传入，返回准备好的socket.
即select负责IO等待，当IO准备好了，就将它回返.
这就是select做的事情.
select()说明
select.select(rlist, wlist, xlist[, timeout])
rlist 等待直到准备好读--为输入而观察的文件对象列表
wlist 等待直到准备好写--为输出而观察的文件对象列表
xlist 等待一种意外的情况--观察错误异常的文件列表
timeout 单位是seconds，
默认是timeout=None表示select永远阻塞，直到一个fd准备好
timeout=0 表示不会阻塞
timeout=10 表示最多阻塞10秒，在阻塞期间发现有准备好的文件对象，会立刻退出阻塞状态.
return：返回参数rlist, wlist, xlist中准备好的文件对象，没有则返回([], [], [])
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