embedded trasactions in postgreSQL with python?

[David Rushby]

(I'm also assuming that 'embedded transactions' are equivalent to
nested transactions.)

What about SAPDB ( http://sapdb.org/ )?  It supports nested
transactions, and, in general, more closely approaches the
functionality of Oracle than any other open source RDBMS.  It even
supports an Oracle 7 syntax mode for compatibility.

Importantly, it has official programming interface support for both
Java and Python:
http://sapdb.org/sap_db_program.htm