[Python-ideas] Membership of infinite iterators

Nick Coghlan ncoghlan at gmail.com
Tue Oct 17 07:10:59 EDT 2017

On 17 October 2017 at 19:19, Serhiy Storchaka <storchaka at gmail.com> wrote:

> 17.10.17 09:42, Nick Coghlan пише:
>> On 17 October 2017 at 16:32, Nick Coghlan <ncoghlan at gmail.com <mailto:
>> ncoghlan at gmail.com>> wrote:
>>     So this sounds like a reasonable API UX improvement to me, but you'd
>>     need to ensure that you don't inadvertently change the external
>>     behaviour of *successful* containment tests.
>> I should also note that there's another option here beyond just returning
>> "False": it would also be reasonable to raise an exception like
>> "RuntimeError('Attempted negative containment check on infinite iterator')".
> What about other operations with infinite iterators? min(count()),
> max(count()), all(count(1))? Do you want to implement special cases for all
> of them?

No, as folks expect those to iterate without the opportunity to break out,
and are hence more careful with them when infinite iterators are part of
their application. We also don't have any existing protocols we could use
to intercept them, even if we decided we *did* want to do so.

The distinction I see with "x in y" is:

1. It's pretty easy to write "for x in y in y" when you really meant to
write "for x in y", and if "y" is an infinite iterator, the "y in y" part
will become an unbreakable infinite loop when executed instead of the
breakable one you intended (especially annoying if it means you have to
discard and restart a REPL session due to it, and that's exactly where that
kind of typo is going to be easiest to make)
2. Containment testing already has a dedicated protocol so containers can
implement optimised containment tests, which means it's also trivial for an
infinite iterator to intercept and explicitly disallow containment checks
if it chooses to do so

So the problem is more likely to be encountered due to "x in y" appearing
in both the containment test syntax and as part of the iteration syntax,
*and* it's straightforward to do something about it because the
__contains__ hook already exists. Those two points together are enough for
me to say "Sure, it makes sense to replace the current behaviour with
something more user friendly".

If either of them was false, then I'd say "No, that's not worth the hassle
of changing anything".


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