> On Jun 22, 2017, at 7:29 PM, Cameron Simpson <[email protected]> wrote:
>
>> On 23Jun2017 06:55, Steven D'Aprano <[email protected]> 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 <[email protected]>
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