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

Terry Jan Reedy tjreedy at
Sun Mar 31 21:47:07 CEST 2013

On 3/31/2013 1:57 PM, Byron Ruth wrote:
> I submitted this as bug last night: 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, `` 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_: 
> 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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