change_grade.py~

#!/usr/bin/env python

import re
import fileinput
from sys import stderr,stdout,argv,exit

verbose=False

if len(argv) != 3:
    stderr.write("Usage: change_grade.py STRING VALUE\n")
    exit(1)
    
for line in fileinput.input('-'):
    m = re.search(argv[1], line)
    values = line.rstrip().split(',') #note that split accepts an optional parameter to define what character the string is split on. If none is provided it defaults to any whitespace character
    if m:
        if verbose:
            stderr.write("%r\n" % values)
        values[3] = argv[2] 

    print ",".join(values)