Shuffle for python iterators. This works by holding `bufsize` items back and yielding them sometime later. This is NOT 100% random, proved or anything.

itershuffle.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import random

def itershuffle(iterable,bufsize=1000):
    """Shuffle an iterator. This works by holding `bufsize` items back
    and yielding them sometime later. This is NOT 100% random, proved or anything."""
    iterable = iter(iterable)
    buf = []
    try:
        while True:
            for i in xrange(random.randint(1, bufsize-len(buf))):
                buf.append(iterable.next())
            random.shuffle(buf)
            for i in xrange(random.randint(1, bufsize)):
                if buf:
                    yield buf.pop()
                else:
                    break
    except StopIteration:
        random.shuffle(buf)
        while buf:
            yield buf.pop()
        raise StopIteration


if __name__ == '__main__':
    # demonstration
    print ' '.join(iter('abcdefghijklmnopqrstuv'.upper()))
    for i in range(10):
        print ' '.join(itershuffle('abcdefghijklmnopqrstuv'.upper()))
    
    # no item is lost in process
    for i in range(1000):
        assert set(itershuffle(range(100))) == set(range(100))
    
    # memory usage test. if it was not iterative, we would run out of memory
    #~ for i in itershuffle(xrange(10000000),10):
        #~ pass

Using a heap would probably be faster and easier to reason about.

#!/usr/bin/env python
# -*- coding: utf-8 -*-
def itershuffle(iterable):
  iterable = iter(iterable)
  buf = []

  try:
    while len(buf) < 100:
      buf.append(next(iterable))

    while True:
      i = (random.random() * 100) // 1
      yield buf[i]

      try:
        buf[i] = next(iterable)
      except StopIteration:
        buf.pop(i)
        raise StopIteration

  except StopIteration:
    random.shuffle(buf)

    while buf:
      yield buf.pop()

    raise StopIteration

Inspired by you so I thought I'd share. For python3