#18963: Pattern for object model compatibility isn't friendly for subclasses
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Reporter: aaugustin | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Python 3 | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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.
--
Ticket URL: <https://code.djangoproject.com/ticket/18963>
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