In-place substitution of PyGTK widgets

Question

The code below creates a column of three labels. I would like to take the middle label, and replace it with another widget using the text from the label after the initial creation of the UI.

My actual use case is to take a GTKBuilder populated UI, and replace any particular named label with a dynamically wrapped label at run time. (I used a button here because it's simple but distinct.) Then I can still use Glade to set up the UI, including the labels, and not pepper my Python code with errant labels and strings if I later want to make a label wrap.

The code as it stands does not work - the new button gets added to the end of the column, and I want it to remain in the middle, where label2 was to begin with. What can I do, preferably in wrap_in_button, to make sure it ends up in the correct place? I'd rather keep it general, since the parent may be a Box or a Table or any general Container.

import pygtk
import gtk

def destroy(widget, data=None):
    gtk.main_quit()

def wrap_in_button(label):
    text = label.get_text()
    button = gtk.Button(text)

    parent = label.get_parent()

    if parent:
        parent.remove(label)
        parent.add(button)

def Main():
    # Pretend that this chunk is actually replaced by GTKBuilder work
    # From here...
    window = gtk.Window()
    window.connect('destroy', destroy)

    box = gtk.VBox()
    window.add(box)

    label1 = gtk.Label("Label 1")
    label2 = gtk.Label("Label 2")
    label3 = gtk.Label("Label 3")

    box.pack_start(label1)
    box.pack_start(label2)
    box.pack_start(label3)

    # ...up to here

    # Comment this to see the original layout
    wrap_in_button(label2)

    window.show_all()

    gtk.main()

if __name__ == "__main__":
    Main()

Answer 1

The pygtk docs has some useful information on the add function:

This method is typically used for simple containers such as gtk.Window, gtk.Frame, or gtk.Button that hold a single child widget. For layout containers that handle multiple children such as gtk.Box or gtk.Table, this function will pick default packing parameters that may not be correct.

For containers with more than one child item, it goes on to say that they have specialised functions for adding items. In your case, gtk.Box has three options: pack_start, pack_end, and reorder_child.

I'm not sure how it will handle using pack_start (or pack_end) after an item has been removed. Failing that, reorder_child sounds like it might help.

If none of these work, you could always remove and re-add all widgets in the correct order using pack_start.

Answer 2

Instead of putting the labels directly into the main container you can put each one into it's own box.

Change this:

label1 = gtk.Label("Label 1")
label2 = gtk.Label("Label 2")
label3 = gtk.Label("Label 3")

box.pack_start(label1)
box.pack_start(label2)
box.pack_start(label3)

To this:

box1 = gtk.HBox()
label1 = gtk.Label("Label 1")
box1.pack_start(label1)

box2 = gtk.HBox()
label2 = gtk.Label("Label 2")
box2.pack_start(label2)

box3 = gtk.HBox()
label3 = gtk.Label("Label 3")
box3.pack_start(label3)

box.pack_start(box1)
box.pack_start(box2)
box.pack_start(box3)

The rest of the code can stay the same. You just have to make sure that you only have 1 child widget in those boxes at a time.