How to use sub-window

sub-window.py

import ctypes
import tkinter


if __name__ == "__main__":
    ctypes.windll.shcore.SetProcessDpiAwareness(1)

    #main-window
    root = tkinter.Tk()
    root.title("main")

    #label in main-window
    l = tkinter.Label(
        root,
        text = "Label in main-window"
    )
    l.pack()

    #sub-window
    sub_window = tkinter.Toplevel(root)
    sub_window.title("sub")

    #label in sub-window
    l_s = tkinter.Label(
        sub_window,     #set sub-window as master
        text = "Label in sub-window",
        bg = "lightblue",
    )
    l_s.pack()

    root.mainloop()