Toplevel and Tk Widgets
A Toplevel is roughly like a Frame that is split off into its own window and has additional
methods that allow you to deal with top-level window properties. The Tk widget is
roughly like a Toplevel, but it is used to represent the application root window.
Toplevel windows have parents, but Tk windows do not—they are the true roots of the
widget hierarchies we build when making tkinter GUIs.
We got a Tk root for free in Example 8-3 because the Label had a default parent, des-
ignated by not having a widget in the first argument of its constructor call:
Label(text='Popups').pack() # on default Tk() root windowPassing None to a widget constructor’s first argument (or to its master keyword argu-
ment) has the same default-parent effect. In other scripts, we’ve made the Tk root more
explicit by creating it directly, like this:
root = Tk()
Label(root, text='Popups').pack() # on explicit Tk() root window
root.mainloop()In fact, because tkinter GUIs are a hierarchy, by default you always get at least one Tk
root window, whether it is named explicitly, as here, or not. Though not typical, there
may be more than one Tk root if you make them manually, and a program ends if all
its Tk windows are closed. The first Tk top-level window created—whether explicitly
by your code, or automatically by Python when needed—is used as the default parent
window of widgets and other windows if no parent is provided.
You should generally use the Tk root window to display top-level information of some
sort. If you don’t attach widgets to the root, it may show up as an odd empty window
when you run your script (often because you used the default parent unintentionally
in your code by omitting a widget’s parent and didn’t pack widgets attached to it).
Technically, you can suppress the default root creation logic and make multiple root
windows with the Tk widget, as in Example 8-4.
Example 8-4. PP4E\Gui\Tour\toplevel1.py
import tkinter
from tkinter import Tk, Button
tkinter.NoDefaultRoot()
win1 = Tk() # two independent root windows
win2 = Tk()
Button(win1, text='Spam', command=win1.destroy).pack()
Button(win2, text='SPAM', command=win2.destroy).pack()
win1.mainloop()
When run, this script displays the two pop-up windows of the screenshot in Fig-
ure 8-3 only (there is no third root window). But it’s more common to use the Tk root
as a main window and create Toplevel widgets for an application’s pop-up windows.
Top-Level Windows | 421