Making a python iterator go backwards?

Am I missing something or couldn't you use the technique described in the Iterator section in the Python tutorial?

>>> class reverse_iterator:
...     def __init__(self, collection):
...         self.data = collection
...         self.index = len(self.data)
...     def __iter__(self):
...         return self
...     def next(self):
...         if self.index == 0:
...             raise StopIteration
...         self.index = self.index - 1
...         return self.data[self.index]
...     
>>> for each in reverse_iterator(['a', 1, 2, 3, 'c', 17]):
...     print each
... 
17
c
3
2
1
a

I know that this doesn't walk the iterator backwards, but I'm pretty sure that there is no way to do that in general. Instead, write an iterator that walks a discrete collection in reverse order.

Edit you can also use the reversed() function to get a reversed iterator for any collection so that you don't have to write your own:

>>> it = reversed(['a', 1, 2, 3, 'c', 17])
>>> type(it)
<type 'listreverseiterator'>
>>> for each in it:
...  print each
... 
17
c
3
2
1
a

No, in general you cannot make a Python iterator go backwards. However, if you only want to step back once, you can try something like this:

def str(self, item):
    print item

    prev, current = None, self.__iter.next()
    while isinstance(current, int):
        print current
        prev, current = current, self.__iter.next()

You can then access the previous element any time in prev.

If you really need a bidirectional iterator, you can implement one yourself, but it's likely to introduce even more overhead than the solution above:

class bidirectional_iterator(object):
    def __init__(self, collection):
        self.collection = collection
        self.index = 0

    def next(self):
        try:
            result = self.collection[self.index]
            self.index += 1
        except IndexError:
            raise StopIteration
        return result

    def prev(self):
        self.index -= 1
        if self.index < 0:
            raise StopIteration
        return self.collection[self.index]

    def __iter__(self):
        return self