Исключение для пятнашек

import tkinter as tk, random as rnd
from tkinter import messagebox as mb
from tkinter import simpledialog as sd
 
root = tk.Tk()
 
field = []
 
 
def set_nums():
    global field
    captions = [str(i) for i in range(1,16)] + ['']
    rnd.shuffle(captions)
    #print(captions)
    for i in range(len(field)):
        field[i]['text'] = captions[i]
        
 
 
def set_field():
    global field
    for i in range(4):
        for j in range(4):
            field.append(tk.Label(root, font=('Arial', 20), text='', width=6,
                                  height=3, borderwidth=1, relief='solid'))
            field[-1].grid(row=i, column=j)
 
 
def is_victory():
    for i in range(len(field)-1):
        if field[i]['text'] != str(i + 1):
            return False
    return True
 
 
def cheeting():
    global field
    for i in range(12):
        field[i]['text'] = str(i+1)
    field[12]['text'] = ''
    field[13]['text'] = '13'
    field[14]['text'] = '14'
    field[15]['text'] = '15'
 
def keypress(e):
    global field
    empty = get_empty_index()
    if e.keycode == 40 and empty > 3: #up
        field[empty]['text'], field[empty-4]['text'] = field[empty-4]['text'], field[empty]['text']
    if e.keycode == 38 and empty < 12: #down
        field[empty]['text'], field[empty + 4]['text'] = field[empty + 4]['text'], field[empty]['text']
    if e.keycode == 39 and empty % 4 != 0: #right
        field[empty]['text'], field[empty - 1]['text'] = field[empty - 1]['text'], field[empty]['text']
    if e.keycode == 37 and empty % 4 != 3: #left #empty not in [3, 7, 11, 15]
        field[empty]['text'], field[empty + 1]['text'] = field[empty + 1]['text'], field[empty]['text']
    if e.keycode == 32:cheeting()
    if is_victory():
        mb.showinfo('Победа!', 'Ура, товарищи!')
        
 
def get_empty_index():
    for i in range(len(field)):
        if field[i]['text'] == '':
            return i
 
 
 
 
 
 
set_field()
set_nums()
root.bind('<Key>', keypress)
 
root.mainloop()