Metakit is an interesting embedded database engine. It isn't strictly relational, but draws heavily on relational concepts. Where relational databases talk about tables of rows and columns, Metakit has views of rows and properties. They seem isomorphic except that a property can itself be a subview, like another table inside a single row. It includes bindings for a number of languages, including one for Python. There's an implementation of SQL for it, if you are more comfortable working in that query language.