[Python-ideas] Membership of infinite iterators

[Paul Moore]

OK, looks like I've lost track of what this thread is about then.
Sorry for the noise.
Paul

On 18 October 2017 at 16:40, Koos Zevenhoven <k7hoven at gmail.com> wrote:
> 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 +