Should I use "if" or "try" (as a matter of speed)?

[Steve Juranich]

I know that this topic has the potential for blowing up in my face,
but I can't help asking.  I've been using Python since 1.5.1, so I'm
not what you'd call a "n00b".  I dutifully evangelize on the goodness
of Python whenever I talk with fellow developers, but I always hit a
snag when it comes to discussing the finer points of the execution
model (specifically, exceptions).

Without fail, when I start talking with some of the "old-timers"
(people who have written code in ADA or Fortran), I hear the same
arguments that using "if" is "better" than using "try".  I think that
the argument goes something like, "When you set up a 'try' block, you
have to set up a lot of extra machinery than is necessary just
executing a simple conditional."

I was wondering how true this holds for Python, where exceptions are
such an integral part of the execution model.  It seems to me, that if
I'm executing a loop over a bunch of items, and I expect some
condition to hold for a majority of the cases, then a "try" block
would be in order, since I could eliminate a bunch of potentially
costly comparisons for each item.  But in cases where I'm only trying
a single getattr (for example), using "if" might be a cheaper way to
go.

What do I mean by "cheaper"?  I'm basically talking about the number
of instructions that are necessary to set up and execute a try block
as opposed to an if block.

Could you please tell me if I'm even remotely close to understanding
this correctly?
-- 
Steve Juranich
Tucson, AZ
USA