[Python-ideas] Block-Scoped Exception Handlers

[Nick Coghlan]

On 5 May 2016 9:16 am, "Kyle Lahnakoski" <klahnakoski at mozilla.com> wrote:
>
>
> On 5/4/2016 7:05 PM, cs at zip.com.au wrote:
> > I also like context from close to where the exception occurred, while
> > doing that catching at whatever the suitable outer layer may be, as
> > normal. Let me show you what I do...
> >
> > I have a module "cs.logutils":
> >
>
> I really like the idea of using a `with` clause for simplify exception
> chaining.  I am concerned I would be missing some of the locals
> available at exception time, which the `with` clause would not have
> access to, but more introspection may solve that too.  It does not solve
> the indent problem, but I could live with that if it made the code
> simpler in other ways.
>
> Many of my exception handlers are multiline, and I do not think the
> `with` clause strategy would work there.

In combination with contextlib.contextmanager (and perhaps passing in
references to relevant locals), with statements are designed to handle
factoring out almost arbitrary exception handling. Chaining a consistent
error, for example:

@contextmanager
def chain_errors(exc_to_raise):
    try:
        yield
    except Exception as e:
        raise exc_to_raise from e

This is most useful for supplying state that's useful for debugging
purposes (e.g. the codec infrastructure tries to do something like that in
order to report the codec name)

Cheers,
Nick.

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