Downsample lines in a file, useful for csv's that are too big.

downsample.py

#!/bin/python
'''
Usage:
python downsample.py [offset+]amount
Examples:
cat super_big.csv | python downsample.py 1+4 > big_divided_by_4.csv
cat data.csv | python downsample.py 2 > data_halved.csv
'''

import sys
from itertools import islice

try:
  if len(sys.argv) != 2:
    raise Exception("Accepts one and only one argument")
  args = sys.argv[1].split('+')
  if len(args) == 2:
    offset, amount = map(int, args)
  elif len(args) == 1:
    offset, (amount,) = 0, map(int, args)
  else:
    raise Exception("Only one `+` should be specified")
  for line in islice(sys.stdin, offset, None, amount):
    sys.stdout.write(line)
except Exception as e:
  print(e, file=sys.stderr)
  print('Must specify downsample amount ([offset+]amount)', file=sys.stderr)