[Python-ideas] Introduce collections.Reiterable

[Nick Coghlan]

On 23 September 2013 18:55, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:
> On 23 September 2013 09:44, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> The fact Python 3 lets you create ranges you can't directly take the
>> length of is a bit of a hack, too (because of the pain involved in
>> defining an alternative __len__ protocol that didn't funnel everything
>> through an ssize_t value):
>>
>>>>> x = range(10**100)
>>>>> len(x)
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> OverflowError: Python int too large to convert to C ssize_t
>>>>> (x.stop - x.start) // x.step
>> 10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
>
> I wouldn't call that a "hack". It's clearly a bug.

The hack is that we were able to make those large ranges possible
*without* fixing the limitation that len() (or, more accurately, the
CPython tp_len slot) only supports 64-bit containers. Solving the
latter is a *much* harder problem that would require a PEP to add a
new type slot, and it's hard to justify doing all that work for such a
niche use case, especially when the workaround is relatively simple.

If we'd taken the purist approach, then the result would more likely
have been that ranges would have remained limited to lengths that fit
in 64 bits rather than that the 64-bit limitation would have been
removed.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia