import itertools as itl

import numpy as np

k = 10

def sample_after(stream, k):
    """
    Return a random sample ok k elements drawn without replacement from stream.
    
    This function is designed to be used when the elements of stream cannot
    fit into memory.
    """
    r = np.array(list(itl.islice(stream, k)))
    
    for t, x in enumerate(stream, k + 1):
        i = np.random.randint(1, t + 1)

        if i <= k:
            r[i - 1] = x
    
    return r

sample_after(xrange(1000000000), 10)