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

[Thorsten Kampe]

* Steve Juranich (2005-07-09 19:21 +0100)
> 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.

"Catch errors rather than avoiding them to avoid cluttering your code
with special cases. This idiom is called EAFP ('easier to ask
forgiveness than permission'), as opposed to LBYL ('look before you
leap')."

http://jaynes.colorado.edu/PythonIdioms.html