Getting the value of a checkbox

This is code I found when searching to understand and learn about Tkinter, but it gives an error on check box toggle.

from Tkinter import * 
    class Example(Frame):


        def __init__(self, parent):
            Frame.__init__(self, parent)   
            self.parent = parent
            self.initUI()

        def initUI(self):      
            self.parent.title("Windows")


            Label(text="Contact List").grid(row=0,column=0,columnspan=2)
            Text(width=30,height=15).grid(row=1,rowspan=9, column=0,columnspan=2,padx=20)
            Button(text="Display Contact").grid(row=10, column=0,columnspan=2,pady=10)
            Label(text="Last Name:").grid(row=11, column=0,pady=10)
            Entry().grid(row=11,column=1)
            Button(text="Search").grid(row=12,column=0,columnspan=2)
            Label(text="New Contact").grid(row=0,column=2,columnspan=2)
            Label(text="First Name:").grid(row=1,column=2,sticky=E)
            Entry().grid(row=1,column=3)
            Label(text="Last Name:").grid(row=2,column=2,sticky=E)
            Entry().grid(row=2,column=3)
            Label(text="Phone #:").grid(row=3,column=2,sticky=E)
            Entry().grid(row=3,column=3)

            friend_check = IntVar()
            Checkbutton(variable=friend_check, command = self.friend_box, onvalue=1, offvalue=0, text = "Friend").grid(row=4,column=3,sticky=W)

            #Label(text="Friend").grid(row=4,column=3,padx=20,sticky=W)
            Label(text="Email:").grid(row=5,column=2,sticky=E)
            Entry().grid(row=5,column=3)
            Label(text="Birthday:").grid(row=6,column=2,sticky=E)
            Entry().grid(row=6,column=3)
            Button(text="Add Contact").grid(row=7,column=3,sticky=E)


        def friend_box(self):

            if self.friend_check.get() == 1:
                print '1'
            else:
                print '0'


    def main():

        root = Tk()

        root.geometry("600x450+900+300")
        root.resizable(0,0)
        app = Example(root)
        root.mainloop()  


    if __name__ == '__main__':
        main()  

This is the error mentioned above:

AttributeError: Example instance has no attribute 'friend_check'

Exception in Tkinter callback

How can I avoid this error?