'MessageDialog' object has no attribute 'secondary_text'
The documentation on Gtk.MessageDialog states this as well as other other properties as readable and writeable, but in fact, it doesn't even seem to exist. It works when used in the constructor, but not afterwards. I even tried getattr(Gtk.MessageDialog(), "secondary-text", str) in case there's something going on with the dash, but same result.
If this is an issue with Gtk.MessageDialog it might be an issue with other widgets as well.