Conditionally implementing __iter__ in new style classes

[Thomas Heller]

bokr at oz.net (Bengt Richter) writes:

> On Thu, 07 Jul 2005 09:51:42 +0200, Thomas Heller <theller at python.net> wrote:
>
>>bokr at oz.net (Bengt Richter) writes:
>>
>>> On Wed, 06 Jul 2005 17:57:42 +0200, Thomas Heller <theller at python.net> wrote:
>>>
>>>>I'm trying to implement __iter__ on an abstract base class while I don't
>>>>know whether subclasses support that or not.
>>
>>> Will a property or custom descriptor do what you want? E.g.
>>>
>>>  >>> class Base(object):
>>>  ...     def __getIter(self):
>>>  ...         if hasattr(self, "Iterator"):
>>>  ...             return self.Iterator
>>>  ...         raise AttributeError, name
>>>  ...     __iter__ = property(__getIter)
> [...]
>>
>>Yep, that's exactly what I need - thanks.
>>
> BTW, I forgot to mention that you could use property as a decorator
> in the above single-argument case:
>
>  >>> class Base(object):
>  ...     @property
>  ...     def __iter__(self):
>  ...         if hasattr(self, "Iterator"):
>  ...             return self.Iterator
>  ...         raise AttributeError, name

Of course.  I didn't spot this, but I cannot use this anyway for 2.3
compatibility.

>  ...
>  >>> class Concrete(Base):
>  ...     def Iterator(self):
>  ...         yield 1
>  ...         yield 2
>  ...         yield 3
>  ...
>  >>> iter(Base())
>  Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
>  TypeError: iteration over non-sequence
>  >>> iter(Concrete())
>  <generator object at 0x02EF152C>
>  >>> list(iter(Concrete()))
>  [1, 2, 3]
>
> Hope there isn't a gotcha for your use case in the way an instance attribute
> of the same name is allowed. A custom descriptor could eliminate that.
>
>  >>> inst = Concrete()
>  >>> list(iter(inst))
>  [1, 2, 3]
>  >>> inst.__init__ = 'abc'
>  >>> list(iter(inst))
>  [1, 2, 3]
>  >>> inst.__init__
>  'abc'

I don't understand what you mean here.  A __iter__ instance attribute?

Thomas