YouTube subtitle converter (download format -> .sub)

#!/usr/bin/env python

from __future__ import print_function

import re
import sys
import datetime

timechars_re = re.compile(r'^[:0-9]+$')


def time_parts(timecode):
    parts = timecode.split(':')
    parts.reverse()
    s = int(parts[0])
    m = int(parts[1])
    h = 0
    if len(parts) >= 3:
        h = parts[2]
    return h, m, s


def time_from(timecode):
    h, m, s = time_parts(timecode)
    return datetime.time(hour=h, minute=m, second=s)


def time_plus(t, hrs_to_add=0, mins_to_add=0, secs_to_add=0, ms_to_add=0):
    assert secs_to_add <= 59
    assert mins_to_add <= 59

    delta = datetime.timedelta(minutes=mins_to_add,
                               seconds=secs_to_add,
                               milliseconds=ms_to_add)

    end = datetime.time(hour=t.hour,
                        minute=t.minute + mins_to_add,
                        second=t.second + secs_to_add,
                        microsecond=t.microsecond + ms_to_add * 1000)
    return end

def start_time(time_text):
    h, m, s = time_parts(time_text)
    t = datetime.time(hour=h, minute=m, second=s)
    t2 = time_plus(t, ms_to_add=100)
    return t2

def end_time(time_text):
    h, m, s = time_parts(time_text)
    t = datetime.time(hour=h, minute=m, second=s)
    return t

def last_time(time_text):
    h, m, s = time_parts(time_text)
    t = datetime.time(hour=h, minute=m, second=s)
    t2 = time_plus(t, secs_to_add=5)
    return t2

def time_line(t0, t1):
    return str(start_time(t0)) + ',' + str(end_time(t1))


def main(fname):
    with open('cc.sub', 'w') as outf:
        print('[SUBTITLE]', file=outf)

        next_time = None
        prev_time = None
        prev_line = None

        with open(fname) as inf:
            for fline in inf:
                fline = fline.strip()
                if timechars_re.match(fline):
                    this_time = fline
                    if prev_time is not None:
                        time_text = time_line(prev_time, this_time)
                        print(time_text, file=outf)
                    if prev_line:
                        print(prev_line, file=outf)
                        print(file=outf)
                    prev_time = this_time
                else:
                    prev_line = fline

            print('%s,%s' % (str(start_time(prev_time)),
                             str(last_time(this_time))), file=outf)
            print(prev_line, file=outf)


if __name__ == '__main__':
    main(sys.argv[1])