pluck.py

#!/usr/bin/python
import random
import sys

if __name__ == "__main__":
    args = sys.argv[1:]
    if args:
        need = int(args[0])
        f = open(args[1], 'r') if len(args) > 1 else sys.stdin
    else:
        print "usage: %s NUM [FILE] - pluck NUM lines from FILE (default stdin)" % args[0]
        sys.exit(0)

    # generalized knuth's shuffle to choose some number of lines to display
    kept = []
    i = 0
    for line in f:
        if i < need:
            kept.append(line)
        else:
            j = random.randint(0, i)
            if j < need:
                kept[j] = line
        i += 1

    for line in kept:
        sys.stdout.write(line)