Python import oddity

Question

Why this snippet:

import tkinter as tk
print(tk.simpledialog.askstring('_', '_'))

produces this error:

AttributeError: module 'tkinter' has no attribute 'simpledialog'

There is a simpledialog module (https://docs.python.org/3/library/dialog.html#module-tkinter.simpledialog). This code works:

from tkinter import simpledialog
print(simpledialog.askstring('_', '_'))