On 2017-06-23 00:29, Cameron Simpson wrote:
On 23Jun2017 06:55, Steven D'Aprano <steve@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.
[snip] Increment a counter on every function call and record it on the exception, perhaps? If the exception's call count == the current call count, it was raised in this function.