A simple python tool to fix time of srt files

fixsrt.py

#!/usr/bin/env python
from sys import argv, exit
from os import path
import datetime

if len(argv) == 5:
    (script, inputf, outputf, sign, msec) = argv
else:
    print "fixsrt <input.srt> <output.srt> <+/-> <milli seconds>"
    exit(1)

def fixline(line, sign, msec):
    times = line.split(' --> ')

    tformat = '%H:%M:%S,%f'
    times[0] = datetime.datetime.strptime(times[0], tformat)
    times[1] = datetime.datetime.strptime(times[1], tformat)

    if sign == '+':
        a = times[0] + datetime.timedelta(milliseconds=msec)
        b = times[1] + datetime.timedelta(milliseconds=msec)
    elif sign == '-':
        a = times[0] - datetime.timedelta(milliseconds=msec)
        b = times[1] - datetime.timedelta(milliseconds=msec)

    a = a.time().strftime("%H:%M:%S,%f")[:12]
    b = b.time().strftime("%H:%M:%S,%f")[:12]

    return '%s --> %s' % (a, b)

if not path.isfile(inputf) and not inputf.endswith('.srt'):
    print "Invalid input file: %s" % inputf
    exit(1)
elif not msec.isdigit() and not len(msec) <= 4:
    print "Invalid time: %s. Pass a numeric value up to 4 digits" % msec
    exit(1)
elif not sign == "+" and not sign == "-":
    print "Invalid Sign: '%s'. Use + or -" % sign
    exit(1)

outf = open(outputf, 'w')

for line in open(inputf, 'r'):
    if '-->' in line:
        line = fixline(line.rstrip(), sign, int(msec))
        print >> outf, line
    else:
        outf.write(line)

outf.close()