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Date: Sun, 31 Mar 2013 17:02:18 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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Subject: Re: collections.Iterator __subclasshook__ does not check if next() is callable
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On 03/31/2013 10:57 AM, Byron Ruth wrote:
> I submitted this as bug last night: http://bugs.python.org/issue17584 and was *honored* to be rejected by Raymond Hettinger. However, I would like feedback on whether my concern (this bug) is justified and clarity if not.
>
> Consider:
>
> ```python
> class A(object):
>      def __init__(self):
>          self.r = iter(range(5))
>      def __iter__(self):
>          return self
>      @property
>      def next(self):
>          return next(self.r)
> ```
>
> The `next` method is a property, however:
>
> ```python
> from collections import Iterator
> a = A()
> isinstance(a, Iterator) # True
> next(a) # TypeError: 'int' object is not callable
> ```
>
> I am using `collections.Iterator` as the means to check if the object is an iterator, however I am not sure if that is _root_ problem here. My understanding of the iterator protocol is that is assumes the __iter__ and next *methods* are implemented. In the example, `A.next` is defined as a property, but is still identified as an iterator. To me, this is incorrect behavior since it's not conforming to the iterator protocol requirements (i.e. a `next` method, not a property).

The root problem is that whoever implemented A did so incorrectly.


> Raymond stated: "The design of ABCs are to check for the existence to required named; none of them verify the signature." I think I understand _why_ this is the case.. but I downstream libraries use `collections.Iterator` to determine if an object _is one_: see https://github.com/django/django/blob/master/django/utils/itercompat.py#L22-L31
>
> Who's job is it to check if `next` (and technically `__iter__`) are methods?

The programmer who wrote it, with good tests.

Technically, the issue is not that 'next' is a property, but that it is not returning a callable.  If it did (and the 
callable was appropriate for the iterator), that would also work.

--
~Ethan~