[Python-3000] Exception Expressions (original) (raw)

Brett Cannon brett at python.org
Thu Aug 31 20:20:20 CEST 2006

On 8/31/06, Calvin Spealman <ironfroggy at gmail.com> wrote:

I thought I felt in the mood for some abuse today, so I'm proposing something sure to give me plenty of crap, but maybe someone will enjoy the idea, anyway.

Never hurts too much to try, huh? =) Plus it gives me a break from my work.

This is a step beyond the recently added conditional

expressions. I actually made this up as a joke, explaining at which point we would have gone too far with branching logic in an expression. After making the joke, I was sad to realize I didn't mind the idea and thought I'd see if anyone else doesn't mind it either.

expr1 except expr2 if exctype For example, given a list, letters, of ['a', 'b', 'c'], we would be able to do the following: print letters[7] except "N/A" if IndexError

So this feels like the Perl idiom of using die: open(file) or die (or something like that; I have never been a Perl guy so I could be off).

This would translate to something along the lines of:

try: tmp = letters[7] except IndexError: tmp = "N/A" print tmp Obviously, the except in an expression has to take precedence over if expressions, otherwise it would evaluate '"N/A" if IndexError" first. The syntax can be extended in some ways, to allow for handling multiple exception types for one result or different results for different exception types: foo() except "Bar or Baz!?" if BarError, BazError foo() except "Bar!" if BarError, "Baz!" if BazError Other example use cases: # Fallback on an alternative path open(filename) except open(filename2) if IOError # Handle divide-by-zero while expr != "quit": print eval(expr) except "Can not divide by zero!" if ZeroDivisionError expr = rawinput() # Use a cache when an external resource timesout db.get(key) except cache.get(key) if TimeoutError Only very basic exception handling would be useful with this syntax, so nothing would ever get out of hand, unless someone wasn't caring about their code looking good and keeping good line lengths, so their code probably wouldn't look great to begin with.

The problem I have with this whole proposal is that catching exceptions should be very obvious in the source code. This proposal does not help with that ideal. So I am -1 on the whole idea.

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