Thomas Heller <[EMAIL PROTECTED]> writes: > 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?
I forgot to mention this: The Base class also implements a __getitem__ method which should be used for iteration if the .Iterator method in the subclass is not available. So it seems impossible to raise an exception in the __iter__ method if .Iterator is not found - __iter__ MUST return an iterator if present. Thomas -- http://mail.python.org/mailman/listinfo/python-list
