updating before insert

I've tried a couple different ways of doing this but I'm screwing
something up.  I know this is part SQL but it's also part Python and how
they interfere... I mean interacts with each other.

What's the right way to update a few fields in a record if the record
exists otherwise, create the record?

Related is:

What's the right way to test for a record (including a predicate against
some of the fields (specifically timestamp calculations)) and if the
record doesn't exist, add it.

In both cases, handle the implicit race conditions between testing and
creating/setting.

(I'm coming to feel like that Monty Python character, sitting in a local
restaurant screeching "but I don't like SQL")

---eric
(related to those bloody Vikings)


-- 
Speech-recognition in use.  It makes mistakes, I correct some.