In a procedure called from the top level or an anonymous code block
   (DO command) called from the top level it is possible to
   control transactions.  To commit the current transaction, call
   plpy.commit().  To roll back the current transaction,
   call plpy.rollback().  (Note that it is not possible to
   run the SQL commands COMMIT or
   ROLLBACK via plpy.execute or
   similar.  It has to be done using these functions.)  After a transaction is
   ended, a new transaction is automatically started, so there is no separate
   function for that.
  
Here is an example:
CREATE PROCEDURE transaction_test1()
LANGUAGE plpythonu
AS $$
for i in range(0, 10):
    plpy.execute("INSERT INTO test1 (a) VALUES (%d)" % i)
    if i % 2 == 0:
        plpy.commit()
    else:
        plpy.rollback()
$$;
CALL transaction_test1();
Transactions cannot be ended when an explicit subtransaction is active.