class Changable:
  def __init__(self):
    self.listeners = []

  def add_listener(listener):
    self.listeners.append(listener)
    
  def change():
    for listener in self.listeners:
      listener.on_change(self)
  

class Class1:
  def __init__(self, changable):
    changable.add_listener(self)

  def on_change(changable):
    pass

changable = Changable()
obj1 = Class1(changable)
obj2 = Class1(changable)

changable.change()