This also affects xrange in Python2, so I chose the affected version as python27.

range object (and xrange in Python2) has __len__(), but the range_iterator object created from __reversed__() doesn't have __len__.

Python2:
>>> x = xrange(10)
>>> len(x)
10
>>> reversed(x)
<rangeiterator object at 0x7f86065684e0>
>>> y = reversed(x)
>>> len(y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'rangeiterator' has no len()

Python3.6
>>> x = range(10)
>>> len(x)
10
>>> len(reversed(x))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'range_iterator' has no len()

Hmm I think this also applies to list_listiterator. In general I'd expect these to be true:

l = [1,2,3]
r = range(3)

assert len(l) == len(reversed(l))
assert len(r) == len(reversed(r))

By design, iterators do not have a length (see msg248496).