Making a python iterator go backwards?

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