除非它的顶部有打印语句，否则Python While循环不会继续

Question

因此，我开始使用UDP编写客户端-服务器程序，它可以很好地接收消息，但是采用这种方式，它将在收到并打印出一条消息后关闭服务器应用程序。因此，我决定添加一个while循环，但是问题是，除非我在其顶部有一个print语句，而且我不知道为什么，否则该循环将不会执行。这是到目前为止我双方的代码。

客户：

import socket

# Define address and port
UDP_IP_ADDRESS = "127.0.0.1"
UDP_PORT_NO = 31000

Message = [None] * 200 #Max message of 200 bytes
res = []

clientSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)


# Make sure the user's credentials match possibilities inside userpass.txt
# userpass.txt format: username:password
def checkCreds(usernameInput, passwordInput):
    with open('userpass.txt') as f:
        credentials = [x.strip().split(':', 1) for x in f]

    for username, password in credentials:
        if username == usernameInput and password == passwordInput:
            print "Your credentials match!"
            return True

# Handle a login to send a message
# Main input should be send into main
# Handled in the form of username&password
def login(credentials): 
    userPass = credentials.split("&")

    if (checkCreds(userPass[0], userPass[1])):
        Message[0] = "c"
        Message[1] = "b"
        Message[2] = '\x04'
        Message[3] = raw_input("Message to send: ")

        for val in Message: 
            if val != None : 
                res.append(val) 

        sendMsg = ''.join(map(str, res))
        clientSock.sendto(sendMsg, (UDP_IP_ADDRESS, UDP_PORT_NO))
        del res[:]

# Allows user input to login with specific username and password
# Format: login#username&password
def main():
    while True:
        userInput = raw_input()
        function = userInput.split("#")
        if function[0] == "login":
            login(function[1])


main()

服务器：

import socket, sys
from itertools import islice

# Define address and port
UDP_IP_ADDRESS = "127.0.0.1"
UDP_PORT_NO = 31000

# Set up socket
serverSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Bind the socket
serverSock.bind((UDP_IP_ADDRESS, UDP_PORT_NO))

def main():
    while True:
        #Receive a message and print it out

        data, addr = serverSock.recvfrom(1024)

        print "Message: ",

        for c in islice(data, 3, len(data)):
            sys.stdout.write(c)
        continue

main()