Iterator for Custom Sequence

[Jonathan Hogg]

On 16/7/2002 12:15, in article 87u1mzq5xw.fsf at athens.localdomain, "Patrick
W" <csposting at yahoo.com.au> wrote:

> class LinkedList:
>   ....
>   def __iter__(self):
>       def genitems(ls):
>           n = ls.head
>           while n.data is not None:
>               yield n.data
>               n = n.next
>       return genitems(self)
>   ....
> 
> This does the job, but is it eccentric or counter-Pythonic?  If so,
> what's the conventional way to provide an iterator for a custom
> sequence?

I'm not sure why you've used a nested function. I would have written this
simply as:

    class LinkedList:
        ....
        def __iter__(self):
            node = self.head
            while node.next is not None:
                yield node.data
                node = node.next
        ....

When you call 'iter' on an object it should return an iterator. A generator
returns an iterator when called. So making '__iter__' a generator suffices.
In fact this is the main place I use generators.

I made a couple of changes other than removing the function: I like 'node'
better than 'n', and I think testing if 'node.data' is None is bogus as you
might want to store None in the list. It makes more sense (and is more
normal in linked-lists) to check if 'node.next' is None, since the last item
should have no next item (obviously your other methods would need to match
this convention).

Jonathan