== Nested conditional expressions in Python (and code golf)

Recently I had an occasion to use a nested (or chained) [[conditional
expression
http://docs.python.org/2/reference/expressions.html#conditional-expressions]].
I haven't used conditional expressions much, so at first I just
wrote out what struck me as the obvious way:

.pn prewrap on
>  res = a.field1 if a.field1 else obj.field2 if obj else None

(The goal is to use _a.field1_ if it's got a value, _obj.field2_ if _obj_
is there, and otherwise _None_.)

Then I paused to ask myself if this was going to have the right grouping
of evaluation; testing said that it did, to my pleasant surprise.
It's always nice when Python behaves the way I expected it to and my
naive code works. That it happens so often is one of the reasons that
I like Python so much.

While this nested conditional expression was the obvious way to write
the expression (since I was converting it from what would otherwise be a
nested _if_), it's possible to be more clever. The simple way is to get
rid of the actual conditional expressions in favour of exploiting the
side effects of _or_ and _and_:

>  res = a.field1 or (obj and obj.field2) or None

(Given when I'm trying to do here this doesn't suffer from [[the usual
problem of (ab)using _and_ and _or_ this way MoreAndOrAbuse]].)

Of course we can golf this code further:

>  res = a.field1 or getattr(obj, 'field2', None)

To my mind this is well over the line into excessively clever, partly
because it mixes two different ways to refer to fields in the same
expression. Even the first condensed version is not something I'm
entirely happy with (partly because it's subtly different than the
straightforward version using conditional expressions). So my initial
version is going to stay in my code.

(I think I've basically recanted on [[my views about avoiding
conditional expressions in Python ShouldNotAvoidElse]] by now.
Time moves on and I get used to things.)