Try/except vs. if/else

[Stephen Horne]

On Tue, 23 Sep 2003 11:10:49 -0400, Shu-Hsien Sheu <sheu at bu.edu>
wrote:

>Hi,
>
>In my understanding, using try/except rather than if/else is more 
>pythonic.

When to use an exception can be a difficult issue in any language.

A common suggestion as to when to use them is 'only for errors' - but
what exactly is an error? Really, its just a special case - and if you
are handling that special case it isn't an error any more.

For example, if your program runs out of memory and cannot complete a
requested task, as long as it handles that special case by reporting
that it run out of memory and not by crashing or whatever, it is
probably doing what any decent requirements document would specify -
in case of insufficient memory, report the problem and cleanly abort
the tasks that cannot be completed without terminating or crashing. In
terms of the requirements, no error has occurred - a special case has
simply been handled exactly as per the requirements.

Actually, I'm not really convinced by that argument either. I think I
first read it in Stroustrup, though I can't be bothered checking ATM.
Anyway, what I'm trying to express is that when to use an exception is
more about intuitions and common sense than hard-and-fast rules.

The nearest thing to a hard-and-fast rule, though, is that if you
throw an exception the condition should really be exceptional - a
special case as opposed to a normal case. try/except is not just an
alternative to if/else, and neither is it an alternative to 'break' in
loops (though Icon programmers may be tempted to use it that way).

One major benefit of exceptions is that you don't have to keep
checking 'has any error occured' in multiple if statements in a
function. Any function that does anything even remotely complicated
will have many different possible error conditions. Making sure that
normal-case code is not run when a failure has already occurred can be
a major headache. Enough of a headache that in C, many people think
error handling is the only case where a 'goto' is acceptable (so an
error can trigger a goto straight to the cleanup code at the end of a
function, keeping the rest of the function clean).

There are other issues, of course - exceptions allow your functions to
cope properly with errors that you don't even know about in functions
that you call, for instance.

But basically, if you handle exceptional conditions by raising
exceptions, your logic will get much clearer as for the most part it
only has to deal with the normal case. The exceptional cases get
handled by the exception handlers.

But if you use try/except as an alternative to if/else, people who
have to read your code may well take up dark magics purely so they can
curse you more effectively ;-)


-- 
Steve Horne

steve at ninereeds dot fsnet dot co dot uk