[Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)

Nick Coghlan wrote:
> P.J. Eby wrote:>> At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:>>> [Jeffrey E. McAninch, PhD]>>>> I very often want something like a try-except conditional expression>>>> similar>>>> to the if-else conditional.>>>>>>>> An example of the proposed syntax might be:>>>>    x = float(string) except float('nan')>>>> or possibly>>>>    x = float(string) except ValueError float('nan')>>> +1 I've long wanted something like this.>>> One possible spelling is:>>>>>>   x = float(string) except ValueError else float('nan')>> I think 'as' would be better than 'else', since 'else' has a different>> meaning in try/except statements, e.g.:>>>>    x = float(string) except ValueError, TypeError as float('nan')>>>> Of course, this is a different meaning of 'as', too, but it's not "as">> contradictory, IMO...  ;-)> > (We're probably well into python-ideas territory at this point, but I'll> keep things where the thread started for now)> > The basic idea appears sound to me as well. I suspect finding an> acceptable syntax is going to be the sticking point.> > Breaking the problem down, we have three things we want to separate:> > 1. The expression that may raise the exception> 2. The expression defining the exceptions to be caught> 3. The expression to be used if the exception actually is caught> >>From there it is possible to come up with all sorts of variants.> > Option 1:> > Change the relative order of the clauses by putting the exception> definition last:> >   x = float(string) except float('nan') if ValueError>   op(float(string) except float('nan') if ValueError)> > I actually like this one (that's why I listed it first). It gets the> clauses out of order relative to the statement, but the meaning still> seems pretty obvious to me.> 
A further extension (if we need it):

     result = foo(arg) except float('inf') if ZeroDivisionError else 
float('nan')

The 'else' part handles any other exceptions (not necessarily a good idea!).

or:

     result = foo(arg) except float('inf') if ZeroDivisionError else 
float('nan') if ValueError

Handles a number of different exceptions.

> Option 2:> > Follow the lamba model and allow a colon inside this form of expression:> >   x = float(string) except ValueError: float('nan')>   op(float(string) except ValueError: float('nan'))> > This has the virtue of closely matching the statement syntax, but> embedding colons inside expressions is somewhat ugly. Yes, lambda> already does it, but lambda can hardly be put forward as a paragon of> beauty.> 
A colon is also used in a dict literal.

> Option 3a/3b:> > Raymond's except-else suggestion:> >   x = float(string) except ValueError else float('nan')>   op(float(string) except ValueError else float('nan'))> 
[snip]
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