[Python-ideas] Change how Generator Expressions handle StopIteration

[Guido van Rossum]

I guess implementing groupby() would be a good interview question. :-)

Can we get back to the issue at hand, which is whether and how we can
change the behavior of StopIteraton to be less error-prone?


On Mon, Nov 3, 2014 at 8:10 PM, Joshua Landau <joshua at landau.ws> wrote:

> On 4 November 2014 00:29, Guido van Rossum <guido at python.org> wrote:
> >
> > Regarding Akira Li's examples of groupby(), unfortunately I find both
> > versions inscrutable -- on a casual inspection I have no idea what goes
> on.
> > I would have to think about how I would write groupby() myself (but I'm
> > pretty sure I wouldn't us functools.partial(). :-)
>
> It's worth noting that the functools.partial doesn't really do
> anything. Just changing
>
>     next_value = partial(next, iter(iterable))
>
> to
>
>     iterator = iter(iterable)
>
> and "next_value()" to "next(iterator)" gets rid of it.
>
> I would have tackled it by letting the inner iterator set a "finished"
> flag and I would have exhausted the iterable by iterating over it:
>
>     def groupby(iterable):
>         # Make sure this is one-pass
>         iterator = iter(iterable)
>         finished = False
>
>         # Yields a group
>         def yield_group():
>             nonlocal finished, group_key
>
>             # This was taken off the iterator
>             # by the previous group
>             yield group_key
>
>             for item in iterator:
>                 if item != group_key:
>                     # Set up next group
>                     group_key = item
>                     return
>
>                 yield item
>
>             # No more items in iterator
>             finished = True
>
>         # This is always the head of the next
>         # or current group
>         group_key = next(iterator)
>
>         while not finished:
>             group = yield_group()
>             yield group_key, group
>
>             # Make sure the iterator is exhausted
>             for _ in group:
>                 pass
>
>             # group_key will now be the head of the next group
>
> This does have a key difference. Whereas with groupby you have the
> confusing property that
>
>     from itertools import groupby
>
>     grps = groupby("|||---|||")
>     a = next(grps)
>     b = next(grps)
>     c = next(grps)
>     list(a[1])
>     #>>> ['|', '|', '|']
>
> with mine this does not happen:
>
>     grps = groupby("|||---|||")
>
>     grps = groupby("|||---|||")
>     a = next(grps)
>     b = next(grps)
>     c = next(grps)
>     list(a[1])
>     #>>> []
>
> Was this an oversight in the original design or is this actually
> desired? I would guess it's an oversight.
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-- 
--Guido van Rossum (python.org/~guido)
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