listen_rfid

from threading import Thread
t = Thread(target=self.listen_rfid)
t.daemon = True # this line tells the thread to quit if the GUI (master thread) quits.
t.start()

try:
    # python 2
    import Tkinter as tk
    import ttk
except ImportError:
    # python 3
    import tkinter as tk
    from tkinter import ttk

self.tk = tk.Tk()
self.frame = tk.Frame(self.tk)

#!/usr/bin/env python3
import sys
import select
import MySQLdb

if sys.version_info[0] == 2:
    from Tkinter import *
    import Tkinter as ttk
else:
    from tkinter import *
    import tkinter as ttk

class Fullscreen_Window:
    def __init__(self):
        self.tk = Tk()
        self.frame = Frame(self.tk)
        self.frame.pack()
        ttk.Button(self.tk, text="hello world").pack()

        self.tk.attributes('-zoomed', True)
        self.state = False
        self.tk.bind("<F11>", self.toggle_fullscreen)
        self.tk.bind("<Escape>", self.end_fullscreen)

        print("init running")
        # Schedule self.listen_rfid to run after the mainloop starts
        self.tk.after(0, self.listen_rfid)

    def toggle_fullscreen(self, event=None):
        self.state = not self.state  # Just toggling the boolean
        self.tk.attributes("-fullscreen", self.state)
        print("Toggling")
        return "break"

    def end_fullscreen(self, event=None):
        self.state = False
        self.tk.attributes("-fullscreen", False)
        return "break"

    def listen_rfid(self):
        print("Main loop running")
        dbHost = 'localhost'
        dbName = 'python'
        dbUser = 'python'
        dbPass = 'PASSWORD'

        dbConnection = MySQLdb.connect(host=dbHost, user=dbUser, passwd=dbPass, db=dbName)
        cur = dbConnection.cursor(MySQLdb.cursors.DictCursor)

        # readline is blocking so check that there is input
        # before attempting to read it.
        r, w, x = select.select([sys.stdin], [], [], 0)
        if r:
            # There is available input, so read a line.
            RFID_input = sys.stdin.readline().rstrip()
            cur.execute("SELECT * FROM access_list WHERE rfid_code = '%s'" % (RFID_input))

            if cur.rowcount != 1:
                print("ACCESS DENIED")
            else:
                user_info = cur.fetchone()
                print("Welcome %s!!" % (user_info['name']))

        # keep running every 500 milliseconds for as long as
        # the mainloop is active.
        self.tk.after(500, self.listen_rfid)

if __name__ == '__main__':
    w = Fullscreen_Window()
    w.tk.mainloop()