Socket communicator in python, server in C

Question

I was trying to add a graphic interface to a socket-based communicator in Python3

import sys
import socket
import select
from tkinter import *


def chat_client():
    # initialize GUI
    window = Tk()
    window.geometry("600x800")
    window.title("Communicator")

    w = Canvas(window, width=580, height=450, bg="ivory2")
    w.place(x=5, y=300)

    # whole conversation is stored here
    conversation = Text(window, width=80, height=30, wrap=WORD)
    conversation.place(x=10, y=310)
    label_enter = Label(window, text="Enter message: ")
    label_enter.place(x=5, y=20)

    # the message from here is being taken to send / write on std out
    message_entry = Entry(window, width=30)
    message_entry.place(x=120, y=20)

    msg = ""

    def say():
        x = message_entry.get()
        sys.stdout.write(x)
        conversation.insert(0.0, x + '\n')
        msg = message_entry.get().encode()
        message_entry.delete(0, 'end')


    btn_send = Button(window, text="Send!", command=say).place(x=120, y=50)

    #THIS SEEMS TO BE THE PROBLEM!!!
    window.mainloop()
    ######################

    host = 'localhost'
    port = 8888

    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.settimeout(2)

    # connect to remote host
    try:
        s.connect((host, port))

    except:
        print('Unable to connect')
        sys.exit()

    print('Connected to remote host. You can start sending messages')
    #sys.stdout.write('[Me] ')
    sys.stdout.flush()

    while 1:

        socket_list = [sys.stdin, s]

        # Get the list sockets which are readable
        ready_to_read, ready_to_write, in_error = select.select(socket_list, [], [], 1)

        for sock in ready_to_read:
            if sock == s:
                # incoming message from remote server, s
                data = sock.recv(4096)
                if not data:
                    print('\nDisconnected from chat server')
                    sys.exit()
                else:
                    # print data
                    sys.stdout.write(str(data))
                    #conversation.insert(0.0, str(data) + '\n')
                    sys.stdout.write('\n-> ');
                    sys.stdout.flush()

            else:
                # user entered a message
                #msg = message_entry.get().encode() #sys.stdin.readline().encode()
                s.send(msg.encode())
                msg2 = sys.stdin.readline().encode()
                s.send(msg2)
                sys.stdout.write('\n-> ');
                sys.stdout.flush()
                message_entry.delete(0, 'end')


if __name__ == "__main__":
    sys.exit(chat_client()) 

I'm using THIS server (which works fine). The problem is with Tkinter mainloop() function (line 39) I think. I was moving it to many different places, nothing seems to work. Without the Tkinter window it worked just fine btw, when I open 2 instances of this communicator without tkinter, messages are being sent and received, with tkinter not. Please help, I'm running out of ideas. PS sorry for my bad english, it's not my first language (neither is Python for that matter).