If it helps, I have tons of code that tests for iterators using: iter(obj) is obj That has been a documented requirement for the iterator protocol forever. Its in the PEP. "A class that wants to be an iterator should implement two methods: a next() method that behaves as described above, and an __iter__() method that returns self." https://www.python.org/dev/peps/pep-0234/ We have objects such that: iter(obj) returns an iterator, but aren't themselves iterators. The most common example of that would be, I think, classes that define __iter__ as a generator method: class A: def __iter__(self): for x in range(10): yield x Then we have actual iterators, like iter(A()). They define `__iter__` that returns self. I don't know what I would call an object that only has __next__, apart from "broken" :-( -- Steve