Here's a program that creates a window with a label when run
under IDLE, but violates several rules. How is it working then?

import tkinter
tkinter.Label( text="Hello!" ).pack()

.

- It does not create a main window using Tk(),
- neither does it pass a master to the Label class,
- nor does not call "mainloop" at the end;
and still Python runneth it.

Answers:

All widgets use the last main window (root window) opened as
their master when no master is passed as an argument.
(A program should have only one main window, so replace
"the last main window opened" with "the main window" above.)

When no main window was opened, the widget will create a main
window, and then proceed with the step previously mentioned.

IDLE uses tkinter itself and already has a mainloop. So, the
program will use IDLE's mainloop and therefore has not to
create its own mainloop. (Twenty years ago, it was not even
possibly to call your own "mainloop" under IDLE.)

Also note that we have not bound the label to a name.
Yet it is not garbage collected immediately because the
tkinter library has kept a reference to it!

But, the following program is a more general way of coding,
since it can be extended more easily and does not depend on
IDLE:

import tkinter

root_window = tkinter.Tk()
label = tkinter.Label( root_window, text="Hello!" )
label.pack()
tkinter.mainloop()