collections.Iterator __subclasshook__ does not check if next() is callable
Byron Ruth
bjruth at gmail.com
Sun Mar 31 15:58:15 EDT 2013
Thanks for responding Terry.
I can assure you I did not initially realize both the `next` and the `__iter__` methods were implemented when I ran into my original problem. I saw a behavior and had to work backwards to realize why it was behaving the way it was (the comparison against Iterator). Once I realized this, the behavior made complete sense. It just dawned on me the fact that `next` was not being checked to be callable (I was surprised by this at the time) which is why I investigated the `Iterator.__subclasshook__` and assumed it was behaving incorrectly based on my assumptions.
On Sunday, March 31, 2013 3:47:07 PM UTC-4, Terry Jan Reedy wrote:
> On 3/31/2013 1:57 PM, 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:
>
>
>
> A competent Python programmer should not do that. In Py3, the method is
>
> properly renamed '__next__', which should make doing that accidentally
>
> even less likely.
>
>
>
> >
>
> > ```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,
>
>
>
> Being an Iterator only means that it *might* be 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).
>
>
>
> There is more to any protocol than can be statically checked.
>
>
>
> > Raymond stated: "The design of ABCs are to check for the existence to required named; none of them verify the signature."
>
>
>
> Having the required attributes is currently the definition of being an
>
> instance of an ABC. Adding 'not a property' would be possible. but
>
> hardly worthwhile. Checking signatures would be worthwhile, but
>
> signatures are not yet available to Python for C-coded methods, let
>
> alone other implementations.
>
>
>
> 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, and a user who does not trust the competence of the
>
> programmer. But this is the least of the possible errors.
>
>
>
> --
>
> Terry Jan Reedy
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