Pattern for object model compatibility isn't friendly for subclasses

[Aymeric Augustin]

In the ​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 ​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.

[Aymeric Augustin]

Answer from #python-dev on the last question:

That's the way the interpreter looks up __magic__ methods. They aren't looked up on the object but on the class of the object.

It's safer to mimick the interpreter's behavior closely, so let's adopt the same pattern.

[Aymeric Augustin <aymeric.augustin@…>]

In fc10418fba4fb906e4265650b62c510d526d63f7:

Fixed #18963 -- Used a subclass-friendly pattern

for Python 2 object model compatibility methods.

[Aymeric Augustin <aymeric.augustin@…>]

In d7688a010a34033a5cc8eecf7b1460169c0e056e:

[1.5.x] Fixed #18963 -- Used a subclass-friendly pattern

for Python 2 object model compatibility methods.

Backport of fc10418 from master.