Take an .srt file and split the text line if the character count goes above a certain threshold.

split_subtitle_file_into_multiline.py

import sys
import argparse

"""
Example usecase:

python split_srt_lines.py SUBTITLE_FILE_PATH.srt -o OUTPUT_FILE_PATH.srt --max_line_length=42 --comma_split_percent=75

if you don't specify an output path, it will replace the file content of the input file

I would not recommend running this multiple times on the same file, as this script currently does not support reprocessing, and might introduce strange line splits.

"""


def split_text_into_multiline(segment_text, max_line_length, comma_split_threshold):
    
    words = segment_text.split(' ')
    
    lines = [
        words[0]
    ]
    
    for word in words[1:]:
        current_line = lines[-1]
        
        # start a new line if the last word ended with a comma,
        # and we're mostly through this line
        if current_line.endswith(',') and len(current_line) > comma_split_threshold:
            lines.append(word)
            continue
        
        line_with_word = f'{current_line} {word}'
        
        # don't take punctuation into account when checking length
        if len(line_with_word.replace(".", "")) >= max_line_length:
            lines.append(word)
        else:
            lines[-1] = line_with_word
    
    return '\n'.join(lines)


def optional_int(string):
    return None if string == "None" else int(string)


def main():
    parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
    parser.add_argument("subtitle_file", type=str, help=".srt file to split")
    parser.add_argument("--output_file", "-o", type=str, default="", help="output file, if empty will do in-place")
    parser.add_argument("--max_line_length", type=optional_int, default=42, help="max amount of characters for a line in the subtitle files")
    parser.add_argument("--comma_split_percent", type=optional_int, default=75, help="split line on a comma is within this percentage value")
    
    args = parser.parse_args().__dict__
    
    input_file = args.get("subtitle_file")
    
    if args.get("output_file"):
        output_file = args.get("output_file")
    else:
        output_file = input_file
    
    # get the actual integer threshold value
    max_line_length = args.get("max_line_length")
    split_threshold = int(float(max_line_length) * float(args.get("comma_split_percent"))*0.01)
    
    out_lines = []
    with open(input_file, "r") as fp:
        in_lines = fp.readlines()
        
        split_next_line = False
        
        for line in in_lines:
            
            if split_next_line:
                line = split_text_into_multiline(line, max_line_length, split_threshold)
                split_next_line = False
            
            out_lines.append(line)
            
            if "-->" in line:
                split_next_line = True


    with open(output_file, "w+") as fp:
        fp.writelines(out_lines)

if __name__ == "__main__":
    main()

see this whisper discussion for context