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

Byron Ruth bjruth at
Sun Mar 31 19:57:08 CEST 2013

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.


class A(object):
    def __init__(self):
        self.r = iter(range(5))
    def __iter__(self):
        return self
    def next(self):
        return next(self.r)

The `next` method is a property, however:

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, `` 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).

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

Who's job is it to check if `next` (and technically `__iter__`) are methods?

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