Question

How can any gtk.Widget (eg. a progress bar) be put into a gtk.Menu as one of the menu items?

Answer 1

Quoting from the PyGTK documentation:

The gtk.MenuItem and its derived
widget subclasses are the only valid
children of menus.

So the answer is: You can’t. But:

As a gtk.MenuItem is a subclass of gtk.Bin it can hold any valid child widget.

If you create a MenuItem without a label:

item = gtk.MenuItem()

you can add most gtk.Widget subclasses as a child to item.