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\n" % totalbytes
  return 0
if __name__=="__main__":
  main()