If you define your own iterators, there is an incompatibility in the special method name across Py3 and Py2. On Python 3 it is __next__, whereas on Python 2 it is next.

The most elegant solution to this is to derive your custom iterator class from future.builtins.object. This provides a special fallback next method that maps to __next__. Use it as follows:

from future.builtins import object

class Upper(object):
    def __init__(self, iterable):
        self._iter = iter(iterable)
    def __next__(self):                 # Py3-style iterator interface
        return next(self._iter).upper()
    def __iter__(self):
        return self

itr = Upper('hello')
assert next(itr) == 'H'
assert next(itr) == 'E'
assert list(itr) == list('LLO')

This works unless you are defining a subclass of a base class defined elsewhere that does not derive from future.builtins.object.

In this case, you can provide compatibility across Python 2 and Python 3 using the next function in future.builtins:

from some_module import some_base_class

class Upper2(some_base_class):
    def __init__(self, iterable):
        self._iter = iter(iterable)
    def __next__(self):                 # Py3-style iterator interface
        return next(self._iter).upper()
    def __iter__(self):
        return self

from future.builtins import next

itr2 = Upper2('hello')
assert next(itr2) == 'H'
assert next(itr2) == 'E'

next() also works with regular Python 2 iterators with a .next method:

itr3 = iter(['one', 'three', 'five'])
assert 'next' in dir(itr3)
assert next(itr3) == 'one'

This works whenever your code calls the next() function explicitly. If you consume the iterator implicitly in a for loop or list() call or by some other method, the future.builtins.next function will not help; the third assertion below would fail on Python 2:

itr2 = Upper2('hello')

assert next(itr2) == 'H'
assert next(itr2) == 'E'
assert list(itr2) == list('LLO')      # fails because Py2 implicitly looks
                                      # for a ``__next__`` method.

Instead, you can use a decorator called implements_iterator from future.utils to allow Py3-style iterators to work identically on Py2, even if they don't inherit from future.builtins.object. Use it as follows:

from future.utils import implements_iterator

Upper2 = implements_iterator(Upper2)

print(list(Upper2('hello')))
# prints ['H', 'E', 'L', 'L', 'O']

This can of course also be used with the @ decorator syntax when defining the iterator as follows:

@implements_iterator
class Upper2(some_base_class):
    def __init__(self, iterable):
        self._iter = iter(iterable)
    def __next__(self):                 # note the Py3 interface
        return next(self._iter).upper()
    def __iter__(self):
        return self

On Python 3, as usual, this decorator does nothing.