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23.20. How do I update a progress bar and do some work at the same time

You have created a progress bar inside a window, then you start running a loop that does some work:

    while work_left:
        ...do something...
You will notice that the window doesn't even show up, or if it does the progress bar stays frozen until the end of the task. The explanation is simple: gtk is event driven, and you are stealing control away from the gtk main loop, thus preventing it from processing normal GUI update events.

The simplest solution consists on temporarily giving control back to gtk every time the progress is changed:

    while work_left:
        ...do something...
        while gtk.events_pending():
Notice that with this solution, the user cannot quit the application (gtk.main_quit would not work because of new loop [gtk.main_iteration()]) until your heavy_work is done.

Another solution consists on using gtk idle functions, which are called by the gtk main loop whenever it has nothing to do. Therefore, gtk is in control, and the idle function has to do a bit of work. It should return True if there's more work to be done, otherwise False.

The best solution (it has no drawbacks) was pointed out by James Henstridge. It is taking advantage of python's generators as idle functions, to make python automatically preserve the state for us. It goes like this:

    def my_task(data):
        ...some work...
        while heavy_work_needed:
            ...do heavy work here...
            progress_label.set_text(data) # here we update parts of UI
            # there's more work, return True
            yield True
        # no more work, return False
        yield False

   def on_start_my_task_button_click(data):
        task = my_task(data)
The 'while' above is just an example. The only rules are that it should yield True after doing a bit of work and there's more work to do, and it must yield False when the task is done.

More on generators:

Generators are objects that are defined as functions, and when called produce iterators that return values defined by the body of the function, specifically yield statements.

The neat thing about generators are not the iterators themselves but the fact that a function's state is completely frozen and restored between one call to the iterator's next() and the following one. This is just the right thing we need to turn our functions into pseudo-threads, without actually incurring the wrath of the gods of software. More: [www.gnome.org]

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