[Python-ideas] except expression

[Jan Kaliszewski]

18.02.2014 16:56, Paul Moore wrote:

> On 18 February 2014 15:05, Chris Angelico <rosuav at gmail.com> wrote:
>>> Well, yes and no. There are only 2 out of the 10 syntaxes 
>>> originally
>>> proposed in the PEP at the start of this thread that use a colon. 
>>> I've
>>> already pointed out that I don't like a colon, so assume I said
>>> "except return 0" if you must :-)
>>
>> Whether it's a colon or another keyword, _something_ is necessary in
>> there, if it's permissible to specify an exception type:
>>
>> sum(x[3] except IndexError 0 for x in list_of_tuples)
>> sum(x[3] except 0 for x in list_of_tuples)
>>
>> How would you parse each of those, without a separator?
>
> With "return" as a separator. I said that above. Was I not clear?

IMHO it would be an abuse of the keyword. "return" is very strong
signal to programmers eyes: "here we exit the body of the function"
(and the function must be defined in the statement-based way: by def,
not by lambda; return is not used in lambda so in an except
expression it would even more alien and out of place).

>> One option might be to have a separator that's present only when the
>> exception type is listed. For instance:
>>
>> sum(x[3] except(IndexError) 0 for x in list_of_tuples)
>> sum(x[3] except 0 for x in list_of_tuples)
>>
>> Does that sort of thing have enough support to be added to the PEP?
>> I'm not sure it gives us anything over the colon notation, which has
>> the benefit of being consistent with the statement form (and is
>> stylistically similar to lambda, so it's not grossly inappropriate 
>> to
>> an expression context)
>
> I remain less than happy with the colon notation, although I will
> concede that the basic
>
>     x[3] except IndexError: 0
>
> form is not horrible - it just becomes horrible very, very fast when
> people try to do anything more complicated than that.
[snip]

IMHO it'll not become horible if some parens are added...

...either after 'except' (my favorite variant):

     x[3] except (IndexError: 0)

     sum(x[3] except (IndexError: 0)
         for x in list_of_tuples)

...or before 'except':

     x[3] (except IndexError: 0)

     sum(x[3] (except IndexError: 0)
         for x in list_of_tuples)

...or just enclosing the whole expression:

     (x[3] except IndexError: 0)

     sum((x[3] except IndexError: 0)
         for x in list_of_tuples)

Cheers.
*j