On 2017-06-23 00:29, Cameron Simpson wrote:
On 23Jun2017 06:55, Steven D'Aprano <st...@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.
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