Opened 6 years ago

Closed 5 years ago

Last modified 5 years ago

#18963 closed Cleanup/optimization (fixed)

Pattern for object model compatibility isn't friendly for subclasses

Reported by: Aymeric Augustin Owned by: Aymeric Augustin
Component: Python 3 Version: master
Severity: Normal Keywords:
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


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.

Change History (3)

comment:1 Changed 6 years ago by Aymeric Augustin

Owner: changed from nobody to Aymeric Augustin
Status: newassigned
Triage Stage: UnreviewedAccepted

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.

comment:2 Changed 5 years ago by Aymeric Augustin <aymeric.augustin@…>

Resolution: fixed
Status: assignedclosed

In fc10418fba4fb906e4265650b62c510d526d63f7:

Fixed #18963 -- Used a subclass-friendly pattern

for Python 2 object model compatibility methods.

comment:3 Changed 5 years ago by 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.

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