[mod_python] Question on Transactions

I am looking for a clean way to create a transaction and use it across 
multiple method calls. For example, method A calls B and C. B updates a 
row successfully but C fails. I like A to abort changes made by B. I 
thought about doing it by passing a connection created in A to B and C 
so I can rollback if needed in A. But I don't like this approach.  I 
like B to creates its own connection but join the transaction that 
already started by A (or somehow get the same connection) or use its own 
transaction context (or a new connection) if none there.

I wonder how other people do this using Python.

Thanks,

-Jalil