TypeError: this constructor takes no arguments

class example3:
    def _init_(self, master):
        self.lbl=Label(master, text="press button below to exit")
        self.lbl.pack()
        self.btn=Button(master, text="quit", command= quit)
        self.btn.pack()

        root=Tk()
        ex3=example3(root)
        root.mainloop()
        def quit(self):

        import sys; sys.exit()