import tkinter as tk

class App:
    def __init__(self, master):
        self.master = master

        self.btn = tk.Button(self.master, text="Click", command=self.fn)
        self.btn.pack()

    def fn(self):
        answer = tk.messagebox.askyesno("Title", "Message")

root = tk.Tk()
myApp = App(root)
root.mainloop()