Conditionally implementing __iter__ in new style classes

[Thomas Heller]

I'm trying to implement __iter__ on an abstract base class while I don't
know whether subclasses support that or not.
Hope that makes sense, if not, this code should be clearer:

class Base:
    def __getattr__(self, name):
        if name == "__iter__" and hasattr(self, "Iterator"):
            return self.Iterator
        raise AttributeError, name

class Concrete(Base):
    def Iterator(self):
        yield 1
        yield 2
        yield 3

The idea is that if a subclass of Base defines an 'Iterator' method,
instances are iterable.  They are not iterable otherwise.

The above gives the expected behaviour: iter(Base()) raises a
"TypeError: iteration over non-sequence", and iter(Concrete()) returns a
generator.

If, however, I make Base a newstyle class, this will not work any
longer.  __getattr__ is never called for "__iter__" (neither is
__getattribute__, btw).  Probably this has to do with data descriptors
and non-data descriptors, but I'm too tired at the moment to think
further about this.

Is there any way I could make the above code work with new style
classes?

Thanks,

Thomas