Soni L. wrote:
> On 2020-04-09 8:48 a.m., Rhodri James wrote:
> > [Muchly snipped]
> > On 09/04/2020 12:27, Soni L. wrote:
> > To put it simple, unpacking raises
> > ValueError:
> > But if the iterator raises ValueError, there's no way to tell it
> > apart from the unpacking:
> > I don't see how this is any different from any other case when you get
> > the same exception for different errors. If for some reason you
> > really care, subclass ValueError to make a finer-grained exception.
> > And the workaround for this is a bit ugly. We
> > already convert e.g.
> > StopIteration into RuntimeError in many cases, why can't we do so
> > here too?
> > Surely the correct "workaround" is not to do the thing that raises the
> > exception?
> > Technically, I consider it a bug that bugs can shadow API-level
> > exceptions. Any API defining API-level exceptions must carefully control
> > the exceptions it raises. In this case I'd like to use the ValueError
> > from the iterator unpacking API, on my API. But since the iterator
> > unpacking API doesn't control its exceptions, this just does a great job
> > of masking bugs in the code instead.
> > Equally, anything doing computation in __get__ should not propagate
> LookupError except where explicitly intended. And that's not how those
> things are often implemented, unfortunately. There's a reason ppl
> complain so much about certain frameworks "eating all the exceptions".
> They use exceptions as part of their API but let user code raise those
> API-level exceptions, which, because they're part of the API, get
> handled somewhere.
Strictly speaking, there is any unpackaging error in your example. Your example
raises its own ValueError before any unpackaging error is raised.
Indeed
```
x, y = foo()
```
also raises your own `ValueError` and there is any unpackaging error involved.
On the other hand, an alternative design that does not raise any exception,
does raise the proper unpackaging exception. For instance:
```
def foo():
yield True
return
x = foo()
x, = foo()
x, y = foo()
```
outputs:
```
Traceback (most recent call last):
File "...", line 7, in <module>
x, y = foo()
ValueError: not enough values to unpack (expected 2, got 1)
```
So, IMHO, you are mixing two different things here. Am I wrong? Are you talking
about something different? Thank you.
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