Python通过select实现异步IO的方法

#!/usr/bin/env python
import select
#导入select模块
BLKSIZE=8192
def readwrite(fromfd,tofd):
 readbuf = fromfd.read(BLKSIZE)
 if readbuf:
 tofd.write(readbuf)
 tofd.flush()
 return len(readbuf)
def copy2file(fromfd1,tofd1,fromfd2,tofd2):
 ''' using select to choice fds'''
 totalbytes=0
 if not (fromfd1 or fromfd2 or tofd1 or tofd2) :
 #检查所有文件描述符是否合法
 return 0
 while True:
 #开始利用select对输入所有输入的文件描述符进行监视
 rs,ws,es = select.select([fromfd1,fromfd2],[],[])
 for r in rs:
 if r is fromfd1:
 #当第一个文件描述符可读时，读入数据
 bytesread = readwrite(fromfd1,tofd1) 
 totalbytes += bytesread
 if r is fromfd2:
 bytesread = readwrite(fromfd2,tofd2)
 totalbytes += bytesread
 if (bytesread <= 0):
 break
 return totalbytes
def main():
 fromfd1 = open("/etc/fstab","r")
 fromfd2 = open("/etc/passwd","r")
 tofd1 = open("/root/fstab","w+")
 tofd2 = open("/root/passwd","w+")
 totalbytes = copy2file(fromfd1,tofd1,fromfd2,tofd2)
 print "Number of bytes copied %d
" % totalbytes
 return 0
if __name__=="__main__":
 main()