readahead.py

import itertools

def pager(size):
    for i in itertools.count():
        if i % size == 0:
            print 'fetching page:', i / size
        yield i

def readahead(iterable, size=100):
    actual, ahead = itertools.tee(iterable)

    # advance the lookahead iterator 
    for _ in xrange(size):
        next(ahead, None)

    for item in actual:
        yield item
        next(ahead, None)


for item in itertools.islice(readahead(pager(50), 10), 250):
    print item