[Python-ideas] Improving Catching Exceptions

[Andy Dirnberger]

> On Jun 22, 2017, at 7:29 PM, Cameron Simpson <cs at zip.com.au> wrote:
> 
>> On 23Jun2017 06:55, Steven D'Aprano <steve at pearwood.info> wrote:
>>> On Thu, Jun 22, 2017 at 10:30:57PM +0200, Sven R. Kunze wrote:
>>> We usually teach our newbies to catch exceptions as narrowly as
>>> possible, i.e. MyModel.DoesNotExist instead of a plain Exception. This
>>> works out quite well for now but the number of examples continue to grow
>>> where it's not enough.
>> 
>> (1) Under what circumstances is it not enough?
> 
> I believe that he means that it isn't precise enough. In particular, "nested exceptions" to me, from his use cases, means exceptions thrown from within functions called at the top level. I want this control too sometimes.
> 
> Consider:
> 
>   try:
>       foo(bah[5])
>   except IndexError as e:
>       ... infer that there is no bah[5] ...
> 
> Of course, it is possible that bah[5] existed and that foo() raised an IndexError of its own. One might intend some sane handling of a missing bah[5] but instead silently conceal the IndexError from foo() by mishandling it as a missing bah[5].
> 
> Naturally one can rearrange this code to call foo() outside that try/except, but that degree of control often leads to quite fiddly looking code with the core flow obscured by many tiny try/excepts.
> 
> One can easily want, instead, some kind of "shallow except", which would catch exceptions only if they were directly raised from the surface code; such a construct would catch the IndexError from a missing bah[5] in the example above, but _not_ catch an IndexError raised from deeper code such within the foo() function.
> 
> Something equivalent to:
> 
>   try:
>       foo(bah[5])
>   except IndexError as e:
>       if e.__traceback__ not directly from the try..except lines:
>           raise
>       ... infer that there is no bah[5] ...
> 
> There doesn't seem to be a concise way to write that. It might not even be feasible at all, as one doesn't have a way to identify the line(s) within the try/except in a form that one can recognise in a traceback.

How about something like this?

    try:
        val = bah[5]
    except IndexError:
        # handle your expected exception here
    else:
        foo(val)
> 
> 
> Cheers,
> Cameron Simpson <cs at zip.com.au>
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