If the type of x defines __next__, but not __iter__, isinstance(x, collections.Iterator) returns True, but in fact x isn't iterable.

>>> class X:
...     def __next__(self):
...             raise StopIteration()
...
>>> x = X()
>>> isinstance(x, collections.Iterator)
True
>>> issubclass(X, collections.Iterator)
True
>>> list(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'X' object is not iterable

The reason for this is that collections.Iterator.__subclasshook__ checks for a __next__ method, and if finds one, returns True. (The class provides an __iter__ mixin method, so this doesn't cause problems for classes inheriting collections.Iterator.)

A possible solution could be in collections.Iterator.__subclasshook__ checking for both required methods.

> A possible solution could be in 
> collections.Iterator.__subclasshook__ 
> checking for both required methods.

That makes sense.  PEP 234 requires
iterators to support both methods.

Fixed in r86857.
Needs backport.

Alexander, do you want to take care of the backport?

ok

Committed in r86872 (3.1) and r86873 (2.7).