def discard_observer(self, observer):
        """Un-register an observer.

        Args:
            observer: The observer to un-register.

        Returns true if an observer was removed, otherwise False.
        """
        discarded = False
        key = self.make_key(observer)
        if key in self.observers:
            del self.observers[key]
            discarded = True
        return discarded
Un-register an observer.

        Args:
            observer: The observer to un-register.

        Returns true if an observer was removed, otherwise False.