x.py

import sys
import glob


def add_line(filename):
    out = []

    with open(filename) as f:
        for l in f:
            if '-->' in l:
                out.append('{} line:90%\n'.format(l.rstrip()))
            else:
                out.append(l)

    return ''.join(out)


files = glob.glob('*.vtt')


for f in files:
    edited = add_line(f)

    with open(f, 'w') as n:
        n.write(edited)