[Python-ideas] except expression

[Rob Cliffe]

On 19/02/2014 10:46, Rob Cliffe wrote:
>
> On 19/02/2014 08:23, Bruce Leban wrote:
>>
>> On Tue, Feb 18, 2014 at 11:06 PM, Chris Angelico <rosuav at gmail.com 
>> <mailto:rosuav at gmail.com>> wrote:
>>
>>     However, I'm removing two parts of the proposal: ... chaining of
>>
>>     excepts (use parens, "(expr except Exception1: default1) except
>>     Exception2: default2").
>>
>>
>> There are two problems with trying to leave this out.
>>
>> First, you say "use parens" but no use of parenthesis can have the 
>> same effect. The alternative you give:
>>   (expr except E1: default1) except E2: default2
>> is not the same as the chained form. This catches E2 in both the 
>> evaluation of E1 *_and_* the evaluation of default1 while the chained 
>> form catches both exceptions only in the evaluation of E1. And 
>> putting parens anywhere else doesn't do it either.
>>
>> Second, isn't an except expression like any other expression which 
>> can be used anywhere an expression can be? So that means that writing 
>> something that looks like a chained expression without parenthesis 
>> _*will*_ be a valid expression and will have to have some 
>> interpretation (depending on whether except is left associative or 
>> right associative.
> +1.
> I now see that the semantics of
>
> expr1 except E1: (default1 except E2: default2)
>
> and
>
> (expr1 except E1: default1) except E2: default2
>
> are not the same (in an earlier post I thought they were).
>
> And neither of them are the same as:
>
> Evaluate expr1 and see what exception it raises:
>     If it raises E1, return default1
>     If it raises E2, return default2.
>
> (To spell it out, as much for my own benefit as anything else:
>     The first catches E2 in the evaluation of default1.
>     The second catches E2 in the evaluation of expr1 OR default1.
>     The last catches E2 in the evaluation of expr1 only.
> )
> I suggest the form without brackets should (be legal and) mean the 
> last of these, because
>     (1) It feels to me like the natural meaning
>     (2) It can't conveniently be expressed otherwise without 
> introducing additional syntax.
>
> Rob Cliffe
Also (3) When assigned to a variable it is equivalent to
try:
     var = expr1
except E1:
     var = default1
except E2:
     var = default2
>
>>
>> Given that, I think it is better to define it as chained now rather 
>> than leaving people to think it's chained and never being able to add 
>> it in the future (since that will change the semantics of existing 
>> code). This is analogous to handling a < b < c: it's a bug magnet in 
>> C for inexperienced programmers but can't be changed without breaking 
>> code. Starting from scratch in Python, it could be chained.
>>
>> --- Bruce
>> Learn how hackers think: http://j.mp/gruyere-security
>>
>>
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