def get_node_attributes(self, node):
        """Given a node, get a dictionary with copies of that node's
        attributes.

        :param node: reference to the node to retrieve the attributes of.
        :returns: dict -- copy of each attribute of the specified node.
        :raises: ValueError -- No such node exists.

        """
        if not self.has_node(node):
            raise ValueError("No such node exists.")
        attributes = {}
        for attr_name, attr_value in self._node_attributes[node].items():
            attributes[attr_name] = copy.copy(attr_value)
        return attributes
Given a node, get a dictionary with copies of that node's
        attributes.

        :param node: reference to the node to retrieve the attributes of.
        :returns: dict -- copy of each attribute of the specified node.
        :raises: ValueError -- No such node exists.