import tkinter as tk
from tkinter import ttk


root = tk.Tk()
root.title("Parent Window")


def on_open_second_window_clicked():
    second_window = tk.Toplevel(master=root)
    second_window.title("Second Window")

    # Causes the second window to be centered in the parent window
    # amongst other things like: if the parent window gets minimized,
    # the second window will get minimized too.
    second_window.transient(root)

    # In some cases, updating idle tasks is required
    # before doing a grab_set(). Without it, it may show an exception
    # when attempting to use grab_set()
    second_window.update_idletasks()

    # Causes the second window to be on top of the parent window,
    # and it won't allow any interaction with the parent window.
    second_window.grab_set()

    # This will cause this function to not return until
    # the second window has been closed (useful for when waiting
    # for the user's input).
    second_window.wait_window(second_window)


btn_open_second_window = ttk.Button(root,
                                    text="Open Second Window",
                                    command=on_open_second_window_clicked)
btn_open_second_window.pack()


root.mainloop()