modified__get__.py

    def __get__(self, instance, owner):
        """
        Instead of directly returning the value directory, we look up each instance's value using 
        the instance itself as the key.
        """
        if instance is not None: # attribute lookup initiated from instance
            if len(self._instance_values) < NonNullStringDescriptor.CAPACITY:
                self._instance_values[instance] = self.__default
            return self._instance_values.get(instance, self.__default)
        else: # attribute lookup initiated via class
            return self.__default