corecursive fibonacci (source: http://en.wikipedia.org/wiki/Corecursion)

fib.py

from itertools import tee, chain, islice, imap
 
def add(x, y):
    return x + y
 
def fibonacci():
    def deferred_output():
        for i in output:
            yield i
    result, c1, c2 = tee(deferred_output(), 3)
    paired = imap(add, c1, islice(c2, 1, None))
    output = chain([0, 1], paired)
    return result
 
for i in islice(fibonacci(), 20):
    print(i)