postgres.py is a great library. one_or_zero is a bad method name.

It does not return 0.

one_or_none would be better.

Or maybe just one. It's a nice reflection of all.

Or maybe if you want to give Haskellites warm-fuzzies (if they get warm-fuzzies).

Or if you really like one_or_zero then I'll open a ticket to rename all to all_or_nothing. ;-)