hello ,
I use Tkinter , at the line : print modelidvar in Interface3() function , it prints "" , nothing , without any error message , can i know why plz , here is all the code :

# -*- coding: cp1252 -*-
# -*- coding: cp1252 -*-
from Tkinter import *
import Tkinter as tk


#import os
import ttk
import MySQLdb;

#####################################################################################

def center(window):

    sw = window.winfo_screenwidth()
    sh = window.winfo_screenheight()
    rw = window.winfo_reqwidth()
    rh = window.winfo_reqheight()
    xc = (sw - rw) / 3
    yc = (sh -rh) / 3
    window.geometry("+%d+%d" % (xc, yc))
    window.deiconify()


##################################################################################



################################################################################




#############################################################################


def revenir():

           interf3.destroy()
           Interface2()           


#############################################################################

def valider():
        modelidvar = modelid_entry.get()
        print modelidvar
        interf2.destroy()
        Interface3()



#############################################################################    


def Interface3() :

    global interf3
    global modelidvar

    interf3 = Tk()    
    interf3.geometry("500x400")
    interf3.title('                            Base de données ')  

    f1 = Frame(interf3, bg="blue",  width=500, height=400)
    f1.pack( fill=X, expand=0)   


    lab5= Label(interf3, text="NumBon" , bg = "blue", fg = "white" )
    lab5.place ( x=26 , y=100)

    lab6= Label(interf3, text="DateBon" , bg = "blue", fg = "white" )
    lab6.place ( x=85 , y=100)

    lab7= Label(interf3, text="Total/Bon" , bg = "blue", fg = "white" )
    lab7.place ( x=400 , y=100)


    print modelidvar




    brevenir = Button(interf3, text = " revenir ", command = revenir)
    brevenir.place( x=12, y=266)

    Quitter = Button(interf3, text = "Quitter" , command = interf3.destroy)
    Quitter.place( x=230, y=350 )

    interf3.after(0,center,interf3)


    interf3.mainloop()


################################################################################

def Interface2() :

    global interf2

    fenetre.destroy()

    interf2 = Tk()    
    interf2.geometry("500x400")
    interf2.title('                            Base de données ')  

    f1 = Frame(interf2, bg="blue",  width=500, height=400)
    f1.pack( fill=X, expand=0)

    lab2 = Label(interf2, text="Veuillez saisir le numero d'Import du Model ( num+année , ex: 122013 )\n et cliquer sur valider " , bg = "blue", fg = "white" )
    lab2.place ( x=80 , y=45 )

    global modelidvar

    modelidvar = StringVar()

    lab3= Label(interf2, text="Référence Model :" , bg = "blue", fg = "white" )
    lab3.place ( x=30 , y=100)

    modelid_entry = ttk.Entry(interf2, width=13, textvariable=modelidvar)
    modelid_entry.place (x = 140 , y = 100 )

    modelidvar = modelid_entry.get()




    bvalider = Button(interf2, text = " Valider ", command = valider)
    bvalider.place( x=250, y=100)        

    #revenir = Button(interf2, text = " revenir ", command = revenir)
    #revenir.place( x=12, y=266)

    Quitter = Button(interf2, text = "Quitter" , command = interf2.destroy)
    Quitter.place( x=230, y=350 )

    interf2.after(0,center,interf2)



    interf2.mainloop()







################################################################################    


 ############################################################################   





#####################################


fenetre=Tk()
fenetre.geometry("500x400")
fenetre.title('                                     ')

f1 = Frame(fenetre, bg="blue",  width=500, height=400)
f1.pack( fill=X, expand=0)

lab1 = Label(fenetre, text="Gestion de ressources informatiques" , bg = "blue", fg = "white" )
lab1.place ( x=152 , y=25 )

failure_max = 3

global uservar
global passvar


uservar = StringVar()
passvar = StringVar()

Label(fenetre, text = "User name:" , bg = "blue", fg = "white" ).place(x = 66, y = 100)
Label(fenetre, text = "Password:" , bg = "blue", fg = "white" ).place(x = 66, y = 125)

entryu = ttk.Entry(fenetre , textvariable=uservar)
entryu.place (x = 180 , y = 100)

entryp = ttk.Entry(fenetre, show='*', textvariable=passvar)
entryp.place (x = 180 , y = 125)




b = Button(fenetre, borderwidth=2, text="Login", width=7, pady=0, command=Interface2)#, image=photo, compound=CENTER)#, state=DISABLED)
b.place(x = 220 , y = 160)

frame21 = Frame(fenetre , bg="blue",    width=40, height=40)
frame21.pack( fill=X, side = BOTTOM)

buttonquit = Button(fenetre, text = "Quitter", command = fenetre.destroy)        
buttonquit.place( x=224 , y=350 )




##################################



##################################

#Display

fenetre.after(0,center,fenetre)
fenetre.mainloop()

Using global variables is clumsy and leads to errors. You should declare modlidvar as global in function valider() to be consistent.

This question has already been answered. Start a new discussion instead.