Of course, there are other ways of writing this code, but imagine this for
a database interface where a save normally returns the saved object
(inspired by Django)
```
try:
x, y = Foo.save()
except ValueUnpackingError:
// oh... this means saving failed (it returned None instead)
// let's return a sensible response
return ExceptionToResponse(FooStateToException(foo.state))
```
Another example would be an interactive session where you expect an
iterable to have 3 entries (say for a 3D space coordinate) and you want to
check this specifically:
```
try:
x, y, z = point
except ValueUnpackingError:
print(f"point has wrong dimension")
```
where point is an instance of:
```
class Point:
def __iter__(self):
return (float(x) for x in self._internal)
```
Can you spot here why ValueError would result in significantly different
behaviour? ValueError will also catch it if point's internal list contains
a bad entry (e.g. a string that does not convert to a float).
Having these exceptions does not take anything away as they can choose the
more general version.
On Sat, 2 May 2020 at 11:12, Eric V. Smith <e...@trueblade.com> wrote:
> On 5/2/2020 6:00 AM, Henk-Jaap Wagenaar wrote:
>
> On Fri, 1 May 2020 at 10:29, Steven D'Aprano <st...@pearwood.info> wrote:
> [...]
> > "Is it UnpackingOverflowException or PackingUnderflowError or
> > TooManyValuesUnpackException or ValueUnpackingError or ...???"
>
> > It took me far too long to learn the difference between
> > UnicodeEncodingError and UnicodeDecodingError. Or is it
> > UnicodeEncodeError and UnicodeDecodeError?
> >
> > It is possible to have too many fine-grained errors as well as too
> few.
>
> This is thinking about exceptions and the proposal too simply. When I read
> this, I (a) found out that this exception is a built-in so it isn't hard to
> find out which is which (any IDE or IPython would help) but more
> importantly, looking at their MRO, if you cannot understand the difference,
> and you don't have to, you could instead choose to catch UnicodeError
> instead (or the combination of both).
>
> However, if you just want to catch encoding errors you can, and this is
> the crux of this proposal: allowing similar catches throughout the standard
> library as a first class citizen seems sensible to me and I am +1 of this
> proposal.
>
> Please show an example of where you would catch ValueUnpackingError and
> how you'd handle it (recover, re-raise, whatever).
>
> Eric
>
>
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