[Python-ideas] syntax to continue into the next subsequent except block

[Paul Wiseman]

On 17 September 2012 05:49, Steven D'Aprano <steve at pearwood.info> wrote:

> On 17/09/12 10:11, Greg Ewing wrote:
>
>> Guido van Rossum wrote:
>>
>>> The suggestion to add ad-hoc "if <condition>" clauses to random parts
>>> of the syntax doesn't appeal to me at all.
>>>
>>
>> I wouldn't call it a random part of the syntax. This is
>> not like the proposals to add if-clauses to while loops,
>> for loops, etc -- they would just be minor syntactic sugar.
>> This proposal addresses something that is quite awkward to
>> express using existing constructs.
>>
>
> I don't think it is quite awkward. Instead of the proposed:
>
>
> try:
>     something()
> except ValueError as ex if condition(ex):
>     spam()
> except ValueError:
>     # if not condition, fall through to the next except block
>     ham()
>
> this can be easily written as:
>
> try:
>     something()
> except ValueError as ex:
>     if condition(ex):
>         spam()
>     else:
>         ham()
>
> which costs you an indent level, which is not a big deal. (If your
> code is so deeply nested that it is a big deal, it is already in
> desperate need of refactoring.)
>
>
> Instead of the original proposal to add a "continue" statement to
> skip to the next except block:
>
> try:
>     something()
> except HTTPError as ex:
>     if condition(ex): continue  # skip to the next except clause
>     spam()
> except URLError as ex:
>     ham()
>
>
> we can do:
>
>
> try:
>     something()
> except (HTTPError, URLError) as ex:
>     # Technically, we don't even need to list HTTPError, since it
>     # is a subclass it will be caught by URLError too.
>     if type(ex) is URLError or condition(ex):
>         ham()
>     else:
>         spam()
>
>
> Have I missed any suggested use-cases?
>
>
What I think made my original case a bit awkward was a general handler for
a parent exception class, and a specific except clause for a subclass (it
was IOError and Exception originally)

with your ValueError case, say you wanted to call ham on every case of a
parent exception to ValueError (I guess it could only be Exception for this
example), but not ValueError where condition was true,

so how would you write this without the except..if?

try:
    something()
except HTTPError as ex if condition(ex):
    spam()
except URLError:
    # if not condition, fall through to the next except block
    ham()

(I switched ValueError for HTTPError because it has more parent classes
that just Exception, because having 'except Exception' probably isn't the
greatest example)

I don't think any of the existing solutions are that awkward or
> unpleasant to require new syntax. They're actually quite
> straightforward.
>
>
>
> --
> Steven
>
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