collections.Iterator __subclasshook__ does not check if next() is callable

Byron Ruth bjruth at gmail.com
Sun Mar 31 21:58:15 CEST 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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