import sys
import gtk
import gtk.glade
import pango

def gtk_main_quit(*args):
   gtk.main_quit()

def change_font(*args):
   label.set_markup('<big>Text with other fontsize</big>')


window = gtk.Window()
window.set_title("Fonttest")
window.set_border_width(0)

vbox = gtk.VBox(gtk.FALSE, 0)
window.add(vbox) 

label = gtk.Label("Normal text")
vbox.add(label)

button = gtk.Button("Change font of label")
vbox.add(button)
button.connect('clicked', change_font)

button = gtk.Button("Quit")
vbox.add(button)
button.connect('clicked', gtk_main_quit)

window.show_all()
gtk.main()