Substitute Placeholder nodes by its value in tags

def fill_placeholders(self, tags):
        """Substitute Placeholder nodes by its value in tags"""
        def change_p_node_tags(node, children):
            if isinstance(node, Placeholder):
                value = ConstExpr(tags[node.name])
                return value
            else:
                return node.clone(children)

        return map_tree(change_p_node_tags, self)