[Python-ideas] Improving Catching Exceptions

[Chris Angelico]

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.

ChrisA