Python with Gtk3: How to get popupmenu to display?

I have created a class called Menu:

class Menu(Gtk.PopoverMenu):
    def __init__(self):
        Gtk.PopoverMenu.__init__(self)
        # Creates a Gio.MenuModel object
        menu = Gio.Menu.new()
        menu.append('Test 1', 'popup.test1')
        menu.append('Test 2', 'popup.test2')
        # Takes Gio.MenuModel as it's first parameter
        self.bind_model(menu, 'contextMenu')

    def show_menu(self):
        self.popup()

I instantiate this class from another class as follows:

elif event.type == Gdk.EventType.BUTTON_PRESS and event.button == 3:
        popup = Menu()
        popup.show_menu()

This executes without any errors, but no menu displays.

I was expecting a popup menu to display on right-click.

The problem was that I needed to use 'set_relative_to(widget)' where 'widget' is provided by a parameter to the button_press_event. That results in the menu displaying. However, there was a subsequent problem that resulted in menu items being greyed-out despite the fact that they showed as being enabled. To fix that problem, it is necessary that the 2nd parameter of bind_model() be the same as that used in menu.append().

Like this:

self.bind_model(menu, 'popup')