In the regular python shell, you can open Tk windows,
and they will operate even though you didn't call
Tkinter.mainloop(). This is useful, for example, when
you manipulate matplotlib graphs from within the python
shell.
This is done by a hook, installed when a tkapp object
is being initialized, which handles Tk events while the
shell is waiting for user input.

I imitated this behaviour in IDLE: When the subprocess
is idle, it checks whether Tkinter._default_root is set
(this happens when the Tkinter.Tk class, which makes a
tkapp object, is initialized, unless
Tkinter._use_default_root is False), and if so, handles
Tk events.

For me it works very well.

Have a good day,
Noam Raphael