[Python-ideas] Improving Catching Exceptions

[Nick Coghlan]

On 28 June 2017 at 13:16, Chris Angelico <rosuav at gmail.com> wrote:
> On Wed, Jun 28, 2017 at 12:25 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> While generator functions now do that implicitly for StopIteration,
>> and "raise X from Y" lets people write suitable exception handlers
>> themselves, we don't offer an easy way to do it with a context manager
>> (with statement as stack boundary), asynchronous context manager
>> (async with statement as stack boundary), or a function decorator
>> (execution frame as stack boundary).
>>
>> So while I prefer "contextlib.convert_exception" as the name (rather
>> than the "exception_guard" Steven used in his recipe), I'd definitely
>> be open to a bugs.python.org RFE and a PR against contextlib to add
>> such a construct to Python 3.7.
>>
>> We'd have a few specific API details to work out (e.g. whether or not
>> to accept arbitrary conversion functions as conversion targets in
>> addition to accepting exception types and iterables of exception
>> types, whether or not to allow "None" as the conversion target to get
>> the same behaviour as "contextlib.suppress"), but I'm already sold on
>> the general concept.
>
> I agree broadly, but I'm sure there'll be the usual ton of
> bikeshedding about the details. The idea behind this decorator, AIUI,
> is a declaration that "a FooException coming out of here is a bug",
> and if I were looking for that, I'd look for something about the
> function leaking an exception, or preventing exceptions. So maybe
> convert_exception will work, but definitely have a docs reference from
> contextlib.suppress to this ("if exceptions of this type would
> indicate code bugs, consider convert_exception instead"). In my
> testing, I've called it "no_leak" or some variant thereon, though
> that's a shorthand that wouldn't suit the stdlib.

Right, and I'd like us to keep in mind the KeyError -> AttributeError
(and vice-versa) use case as well. Similar to ExitStack, it would be
appropriate to make some additions to the "recipes" section in the
docs that covered things like "Keep AttributeError from being
suppressed in a property implementation".

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia