Py3 requirement: chardet

vtt_to_srt.py

import argparse
import datetime
import re
from os.path import exists, splitext, join
from os import walk
import chardet

def main():

    parser = argparse.ArgumentParser()
    parser.add_argument("filename")
    parser.add_argument("-f", "--folder", action="store_true")
    args = parser.parse_args()

    if not exists(args.filename):
        print('Input file missing!')
        return 0

    if args.folder:
        vttFiles = []
        for root, dirs, files in walk(args.filename):
            for name in files:
                f = join(root, name)            
                if splitext(f)[1].lower() == '.vtt':
                    vttFiles.append(f)
    else:
        vttFiles = [args.filename]
    
    for vttFile in vttFiles:
        # Detect file encoding
        with open(vttFile, 'rb') as file:
            raw = file.read() 
            encoding = chardet.detect(raw)['encoding']

        with open(vttFile, encoding=encoding) as f:
            lines = f.readlines()        
            flag = False
            newLines = []
            i = 1
            
            for line in lines:
                if not flag and re.match(r'\d{2}:', line):
                    flag=True
                if flag:
                    if re.match(r'\d{2}:', line):
                        newLines.append(str(i) + '\n' + re.sub(r'(\d{2}:\d{2}:\d{2})\.(\d{3})', r'\1,\2', line))
                        i += 1
                    else:
                        newLines.append(line)


        newFilename = f'{splitext(vttFile)[0]}.srt'

        # Output
        with open(newFilename, 'w', encoding='utf-8') as f:
            f.write(''.join(newLines))

if __name__ == '__main__':
    main()