id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
18963	Pattern for object model compatibility isn't friendly for subclasses	Aymeric Augustin	Aymeric Augustin	"In the [https://docs.djangoproject.com/en/dev/topics/python3/#magic-methods Python 3 documentation] I recommended the following pattern for iterators:

{{{
class MyIterator(object):
    def __iter__(self):
        return self             # implement some logic here

    def __next__(self):
        raise StopIteration     # implement some logic here

    next = __next__             # Python 2 compatibility
}}}

A downside is that if a subclass overrides `__next__`, it also has to repeat `next = __next__`. This is tricky.

As of [20012b96] Django bundles six 1.2, which introduced [http://packages.python.org/six/#six.Iterator six.Iterator] to solve this problem.

This problem cannot cause regressions for people who have working code on Python 2, because they override `next`, not `__next__`. However I'd like to take advantage of `six.Iterator` since it's technically more correct.

----

The same problem exists with `__nonzero__ = __bool__`, `__div__ = __truediv__` and `__idiv__ = __truediv__`.

We could use a slightly more verbose pattern:
{{{
class Foo(object):
    def __nonzero__(self):
        return self.__bool__()
}}}

`six.Iterator` uses `type(self).__next__(self)` rather than `self.__next__()`. I assume there's a good reason but I haven't figured it out. I'd like to understand it before making a decision."	Cleanup/optimization	closed	Python 3	dev	Normal	fixed			Accepted	0	0	0	0	0	0