[Python-ideas] Change how Generator Expressions handle StopIteration

[Yotam Vaknin]

Firstly, sorry for my buggy code that began all this mess.

But what about adding a named parameter to the next function, specifying an exception to be raised on StopIteration, allowing it to propagate?

ב-4 בנוב 2014, בשעה 06:49, Guido van Rossum <guido at python.org> כתב/ה:

> 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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