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

MRAB python at mrabarnett.plus.com
Thu Aug 6 13:39:58 CEST 2009

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 

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


     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'))

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