Returns the specified attribute from the map of all mapped attributes
        for the given mapped class and attribute key. See
        :method:`get_attribute_map` for details.

def get_attribute(self, attribute_name, mapped_class=None, key=None):
        """
        Returns the specified attribute from the map of all mapped attributes
        for the given mapped class and attribute key. See
        :method:`get_attribute_map` for details.
        """
        attr_map = self.__get_attribute_map(mapped_class, key, 0)
        try:
            return attr_map[attribute_name]
        except KeyError:
            raise AttributeError(attribute_name)