[Python-ideas] Generators are iterators

[Oscar Benjamin]

On 12 December 2014 at 22:54, Terry Reedy <tjreedy at udel.edu> wrote:
> On 12/12/2014 4:10 PM, Oscar Benjamin wrote:
>
>> The function was just supposed to generate the effect. More plausible
>> examples can be made. I tripped over this myself once when doing
>> something with iterators that required me to extract the first item
>> before processing the rest. A simple demonstration:
>>
>> def sum(iterable):
>>      iterator = iter(iterable)
>>      total = next(iterator)
>>      for item in iterator:
>>          total += item
>>      return total
>
>
> Examples like this has been posted on python-list, probably more than once.
> The answer has been to catch StopIteration (which should only come from
> next/__next__) and either return a default value or raise ValueError('cannot
> sum an empty iterable').

I know what the solution is. For my purposes simply raising an
exception (any exception that wasn't StopIteration) would have been
sufficient.

I'm just saying that it's a mistake that can easily be made. Since I
made it I'm now super-suspicious of next() so I probably wouldn't make
it again. This mistake is the root of all of the problems that PEP 479
attempts to solve. If you don't believe it is a problem that someone
might make this mistake then it follows that PEP 479 is a massive
mistake.