title: How to deal with "AttributeError: '_tkinter.tkapp' object has no attribute 'messagebox'"?
body: <p>I'm studying about 'tkinter' recently.
  Pycharm told me here is no 'messagebox', but 'messagebox.py' does exist in my 'tkinter'.
  Many ways I gained from Internet can't solve this.
  Please help me, I'll appreciate for that.</p>
  <pre><code>greet_button = Button(
      table,
      text = 'Yes',
      width = 20,
      height = 2,
      bg = 'yellow',
      fg = 'red',
      command = table.messagebox.showinfo(title='Hello', message='Hello!')
  )
  </code></pre>