[Python-ideas] sentinel_exception argument to `iter`

[Ram Rachum]

HI everybody,

I see that discussion has stalled on this suggestion. What can we do to
move this forward?


On Sat, Feb 8, 2014 at 10:32 AM, Terry Reedy <tjreedy at udel.edu> wrote:

> On 2/7/2014 9:28 AM, Serhiy Storchaka wrote:
>
>> 07.02.14 10:36, Terry Reedy написав(ла):
>>
>>> I wrote the above with the idea that there would be a third parameter to
>>> provide an exception. It would have to have a private default like
>>>
>>> class _NoException(Exception): pass
>>>
>>
>> Natural default is StopIteration or ().
>>
>
> That does not work because iter has to know if the user called iter with
> only one argument, an iterable, or with two or (with the proposal) three,
> with the first argument being a callable.
>
> def iter(iter_or_call, sentinel=<private object>, stop_iter=<private
> exception>):
>   if sentinel == <private object> and stop_iter == <private exception>:
>     # iter_or_call must be an iterable
>     <do what iter(iterable) does now,
>      which is to return iterable.__iter__()
>      or getitem_iterable(iterable)>
>   else:
>     # iter_or_call must be a callable
>     return callable_iterator(iter_or_call, sentinel, stop_iter)
>
> where callable_iterator has been modified to take the third parameter and
> raise StopIteration if iter_or_call raises stop_iter.
>
> class Callable_iterator:
>   def __next__(self):
>     try:
>       val = self.func()
>     except self.stop_iter:
>       raise StopIteration from None
>     if self.sentinel == value:
>       raise StopIteration
>     else:
>       return val
>
> If a user passes sentinel but not stop_iter, the except clause should have
> no effect and the method should work as it does currently. A default of
> StopIteration would accomplish that but it does not work for the iter
> switch. A private exception class also works since no exception raised by
> self.func should be an instance of such a class (unless the user violates
> convention).
>
> If a user passes stop_iter but not sentinel, the if clause should have no
> effect and the iteration should continue until there is an exception. So
> the default sentinel should never compare equal to any value returned by
> self.func. A private instance of object on the left of == will invoke
> object.__eq__ which compares by identity. Unless a user violates convention
> by extracting and using the private instance, the function will never
> return it. Since value could compare equal to everything, the order for ==
> matters.
>
> --
> Terry Jan Reedy
>
>
>
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