Python GTK3 multiple UI files

Python GTK3 multiple UI files

I am tying to make a login form using python and Gtk3 with the help of
anjuta. So far I have a ui file for my login section, and a form that
makes an http request. The class looks like this:
class GUI:
def __init__(self):
self.builder = Gtk.Builder()
self.builder.add_from_file(UI_FILE)
self.builder.connect_signals(self)
window = self.builder.get_object('window')
window.set_title("LOGIN")
window.show_all()
self.username = ''
self.password = ''
When I fill in the correct data, the HTTP request is successful and I can
see the response in the console. But what I want to do is clear the
current window(ui file) and load a new one in it's place. That is without
closing the actual window and opening up a new one. How do I do that?