[Python-ideas] Membership of infinite iterators

[Koos Zevenhoven]

On Wed, Oct 18, 2017 at 6:36 PM, Paul Moore <p.f.moore at gmail.com> wrote:

> On 18 October 2017 at 16:27, Koos Zevenhoven <k7hoven at gmail.com> wrote:
> > So you're talking about code that would make a C-implemented Python
> iterable
> > of strictly C-implemented Python objects and then pass this to something
> > C-implemented like list(..) or sum(..), while expecting no Python code
> to be
> > run or signals to be checked anywhere while doing it. I'm not really
> > convinced that such code exists. But if such code does exist, it sounds
> like
> > the code is heavily dependent on implementation details.
>
> Well, the OP specifically noted that he had recently encountered
> precisely that situation:
>
> """
> I recently came across a bug where checking negative membership
> (__contains__ returns False) of an infinite iterator will freeze the
> program.
> """
>
>
​No, __contains__ does not expect no python code to be run, because Python
code *can* run, as Serhiy in fact already demonstrated for another purpose:
​

On Wed, Oct 18, 2017 at 3:53 PM, Serhiy Storchaka <storchaka at gmail.com>
 wrote:

> 18.10.17 13:22, Nick Coghlan пише:
>
>> 2.. These particular cases can be addressed locally using existing
>> protocols, so the chances of negative side effects are low
>>
>
> Only the particular case `count() in count()` can be addressed without
> breaking the following examples:
>
> >>> class C:
> ...     def __init__(self, count):
> ...         self.count = count
> ...     def __eq__(self, other):
> ...         print(self.count, other)
> ...         if not self.count:
> ...             return True
> ...         self.count -= 1
> ...         return False
> ...
> >>> import itertools
> >>> C(5) in itertools.count()
> 5 0
> 4 1
> 3 2
> 2 3
> 1 4
> 0 5
> True



Clearly, Python code *does* run from within itertools.count.__contains__(..)


––Koos​


-- 
+ Koos Zevenhoven + http://twitter.com/k7hoven +
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