How to allow only one instance of a certain class?

Question

If we execute the code below, a window will be created with a button inside, that every clicked will open another window (Window2)... How do I not allow a second instance of Window2? I want to do this without using modal().

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from gi.repository import Gtk

class Main(Gtk.Window):
    def __init__(self):
        button = Gtk.Button('Click')
        Gtk.Window.__init__(self, title = 'Main Window')
        button.connect('clicked', Window2)
        self.connect('delete-event', Gtk.main_quit)
        self.set_default_size(300, 200)
        self.add(button)
        self.show_all()

class Window2(Gtk.Window):
    def __init__(self, widget):
        button = Gtk.Button('Exit')
        Gtk.Window.__init__(self, title = 'Window2')
        button.connect('clicked', Gtk.main_quit)
        self.set_default_size(300, 200)
        self.add(button)
        self.show_all()


if __name__ == '__main__':
    Main()
    Gtk.main()

Answer 1

In the general case, one answer is to use Singleton, as pointed out by Diego Vieira in the comments. In your specific case, I believe that just change the behavior of the button to not create a window every time:

class Main(Gtk.Window):
    def __init__(self):
        self.window2 = None     # Cria-se uma referência no self para a janela

        button = Gtk.Button('Click')
        Gtk.Window.__init__(self, title = 'Main Window')
        button.connect('clicked', self.mostrar_janela)    # Mostra-a quando clicar no botão
        self.connect('delete-event', Gtk.main_quit)
        self.set_default_size(300, 200)
        self.add(button)
        self.show_all()

    def mostrar_janela(self):
        if not self.window2:
            self.window2 = Window2()
        # else mostrar janela ?

Answer 2

I found a solution until feasible for the issue of allowing only one instance of certain window in Gtk

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from gi.repository import Gtk

class Main(Gtk.Window):
    def __init__(self):
        button = Gtk.Button('Click')
        Gtk.Window.__init__(self, title = 'Main Window')
        button.connect('clicked', self.run_window2)
        self.connect('delete-event', Gtk.main_quit)
        self.set_default_size(300, 200)
        self.add(button)
        self.show_all()

    def run_window2(self, widget):
        try:
            #Chama a janela para frente, se a janela ainda não existe,
            #ocorre um AttributeError, passando então para o except e assim criando a janela. 
            self.window2.present()
        except:
            self.window2 = Window2()

class Window2(Gtk.Window):
    def __init__(self):
        button = Gtk.Button('Exit')
        Gtk.Window.__init__(self, title = 'Window2')
        button.connect('clicked', Gtk.main_quit)
        self.set_default_size(300, 200)
        self.add(button)
        self.show_all()


if __name__ == '__main__':
    Main()
    Gtk.main()

Answer 3

Without wanting to belittle the other answers and although they are correct and solve the specific case, maybe in an even better way, also without getting into merits if Singleton is a good design pattern or not my suggestion is for other cases in general.

How to allow only one instance of a given class?

In other words:

How to create a python Singleton?

Using metaclasses:

class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class Window(object):
    __metaclass__ = Singleton

Or in Python 3

class Window(metaclass=Singleton):
    pass

And finally if you want __init__ be called every time the class is called add:

    else:
        cls._instances[cls].__init__(*args, **kwargs)

source: shamelessly inspired by https://stackoverflow.com/questions/6760685/creating-a-singleton-in-python