code: def notify(self, instance, old, new):
          
          if self._disabled.get(instance, False):
              return
          for cback in self._callbacks.get(instance, []):
              cback(new)
          for cback in self._2arg_callbacks.get(instance, []):
              cback(old, new)
docs: Call all callback functions with the current value
  
          Each callback will either be called using
          callback(new) or callback(old, new) depending
          on whether ``echo_old`` was set to `True` when calling
          :func:`~echo.add_callback`
  
          Parameters
          ----------
          instance
              The instance to consider
          old
              The old value of the property
          new
              The new value of the property