hangman tkinter

# -*- coding: utf8 -*-
from tkinter import Frame, Label, Button, Tk, PhotoImage

word = 'apple'
alphabet = 'abcdefghijklmnopqrstuvwxyz'
mistakes_made = 0
max_mistakes = 8
guessed_letters =[]

def word_to_print(word, guessed_letters):
    st=[]
    for letter in word:
        if letter in guessed_letters:
            st.append(letter)
        else:
            st.append('_')
    return ' '.join(st)

def is_word_guessed(word, guessed_letters):
    for letter in word:
        if letter not in guessed_letters:
            return False
    return True

root = Tk()

wordFrame = Frame(root)
wordFrame.pack(side='top', fill='x')

hangstand = Frame(root).pack(side='bottom')
gif = PhotoImage(file='hang1.gif')
hangstandImage = Label(hangstand, image=gif)
hangstandImage.image = gif
hangstandImage.pack()

alphabetFrame = Frame(root)
alphabetFrame.pack(side='bottom',fill='both')

wordLabel = Label(wordFrame, text=word_to_print(word,guessed_letters),
 font=('Arial', 24, 'bold'), relief = 'ridge')
wordLabel.pack()

def draw_mistakes():
    gif = PhotoImage(file='hang'+str(mistakes_made+1)+'.gif')
    hangstandImage.config(image = gif)
    hangstandImage.image = gif

def callback(letter):
    btns[letter].config(state='disabled')

    global mistakes_made
    global guessed_letters

    guessed_letters.append(letter)
    if not letter in word:
        mistakes_made+=1
        draw_mistakes()
    else:
        wordLabel.configure(text=word_to_print(word,guessed_letters))

btns = {}
for letter in alphabet:
    btns[letter] = Button(alphabetFrame, text=letter, padx=3, font=('Arial', 14),
        command = lambda name = letter : callback(name))
    btns[letter].pack(side='left', padx=1)


root.mainloop()