Python：客户端脚本中有多个套接字？

Question

I had this excercise for a class which wanted me to develop a client socket script (in Python 2.x) that connected to a server, received a string and sent back the res variable. The professor straight up told us to look up in the internet about similar scripts and after a relevant search i came up with a base script that worked perfectly for the excercise:

from socket import *
import string, sys, select

HOST = '...'
PORT = 5555

s = socket(AF_INET,SOCK_STREAM)
s.connect((HOST,PORT))

while 1:

 socket_list = [sys.stdin, s]
 read_sockets, write_sockets, error_sockets = select.select(socket_list , [], [])
 for sock in read_sockets:
        if sock == s:
            data = sock.recv(4096)
            if not data : sys.exit()
            else:
              dec = map(ord,data)
              res = max(dec) + min(dec)
              print data+'\n(^min_max value in ascii: '+str(res)+')'
        else :
            msg = sys.stdin.readline()
            s.send(msg)

However, since I'm really new to Python, and the script is based from a random page on the web, I can't fully understand the following:

socket_list = [sys.stdin, s]
read_sockets, write_sockets, error_sockets = select.select(socket_list , [], [])

for sock in read_sockets:

What is happening in these lines? I do understand why I use the two sockets, but the second line confuses me. Any help?

PS: I tried removing that line and changing the loop-statement with

for sock in socket_list

but when i wrote back at the server it didn't accept the right answer

Answer 1

According to what the documentation says about select [1], it returns the sockets ready to read/write. If you do for sock in socket_list you are just picking one socket and trying to read even if that socket is not ready (means it doesn't have any data).

You could try making the steps one by one:

while 1:

    socket_list = [sys.stdin, s]
    #read_sockets, write_sockets, error_sockets = select.select(socket_list , [], [])
    #for sock in read_sockets:
    #if sock == s:

    # read from stdin
    msg = sys.stdin.readline()

    # send string to the server
    s.send(msg)

    # Read from server (it should have data now)
    data = sock.recv(4096)
    if not data:
        sys.exit()

    dec = map(ord,data)
    res = max(dec) + min(dec)
    print data+'\n(^min_max value in ascii: '+str(res)+')'

[1] https://docs.python.org/2/library/select.html#select.select