Here's the nearly shortest possible solution to your question. The solution works in python 3.x. For python 2.x change the import to Tkinter rather than tkinter (the difference being the capitalization):
import tkinter as tk
#import Tkinter as tk # for python 2
def create_window():
window = tk.Toplevel(root)
root = tk.Tk()
b = tk.Button(root, text="Create new window", command=create_window)
b.pack()
root.mainloop()
This is definitely not what I recommend as an example of good coding style, but it illustrates the basic concepts: a button with a command, and a function that creates a window.