It gets a uniform sample of big files

get.uniform.sample.py

#!/usr/bin/python

import os,sys

if len(sys.argv) < 3:
    print "Usage: %s <source file> <percentage>" % sys.argv[0]
    exit(1)

if not os.path.exists(sys.argv[1]):
    print "Provide a file from wich to extract the sample"
    exit(1)

if not (int(sys.argv[2]) < 100 and int(sys.argv[2]) > 1):
    print "Provide a percentage of the sample to take (a number between 1 to 100)"
    exit(1)

srcfile=sys.argv[1]
percentage=float(sys.argv[2])

nlines=0
sf=open(srcfile,"r")
nlines=sum(1 for _ in sf)

nresult=int((float(nlines)*(float(percentage)/100.0)))
step=nlines/nresult

sf.seek(0)
i=0
for line in sf.readlines():
    i+=1
    if (i%step) == 0:
        print line.strip()