Question

Is there anyway to make a python list iterator to go backwards?

Basically i have this

class IterTest(object):
    def __init__(self, data):
        self.data = data
        self.__iter = None

    def all(self):
        self.__iter = iter(self.data)
        for each in self.__iter:
            mtd = getattr(self, type(each).__name__)
            mtd(each)

    def str(self, item):
        print item

        next = self.__iter.next()
        while isinstance(next, int):
            print next
            next = self.__iter.next()

    def int(self, item):
        print "Crap i skipped C"

if __name__ == '__main__':
    test = IterTest(['a', 1, 2,3,'c', 17])
    test.all()

Running this code results in the output:

a
1
2
3
Crap i skipped C

I know why it gives me the output, however is there a way i can step backwards in the str() method, by one step?

EDIT

Okay maybe to make this more clear. I don’t want to do a full reverse, basically what i want to know if there is an easy way to do the equivalent of a bidirectional iterator in python?

Answer 1

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