python's generator methods...

generators.py

from random import shuffle

class Generators(object):
    def __init__(self, n):
        self.a = range(n)

    def __iter__(self):
        for e in self.a:
            yield e

    def backward(self):
        for e in reversed(self.a):
            yield e

    def inorder(self, key=lambda x: x):
        for e in sorted(self.a, key=key):
            yield e

    def randomly(self):
        tmp = self.a[:]
        shuffle(tmp)
        for e in tmp:
            yield e

a = Generators(4)

print 'forward'

for i in a:
    print "%02d" % i

print 'backward'

for i in a.backward():
    print "%02d" % i

print 'inorder'

for i in a.inorder(lambda x: -x):
    print "%02d" % i

print 'forward'

for i in a:
    print "%02d" % i

print 'randomly'

for i in a.randomly():
    print "%02d" % i

print 'backward'

for i in a.backward():
    print "%02d" % i