Sounds like you’ve got a neat project going! Using Python for automation along with `sed` can definitely streamline your workflow. Here’s a quick guide on how you can integrate `sed` into your Python script.

First off, you’re right about using the subprocess module. It’s perfect for running system commands like `sed`. Here’s a simple approach:

import subprocess

# Function to replace text in a file using sed
def replace_in_file(input_file, output_file, search_exp, replacement):
    # Construct the sed command
    command = [
        'sed',
        f's/{search_exp}/{replacement}/g',  # 's/search/replace/g' syntax
        input_file
    ]
    
    # Open the output file
    with open(output_file, 'w') as out_file:
        # Execute the sed command and write the output to the new file
        subprocess.run(command, stdout=out_file, text=True)
    

This script reads your input_file, applies the `sed` command, and writes the results to output_file. Your search pattern should be a valid regex, so if you have special characters, you will need to escape them. For example, if your search expression is a dot (.), you want to use \. in the `sed` command.

Make sure the `sed` command is compatible across platforms. On Windows, you might have to use a compatibility layer like Cygwin or WSL (Windows Subsystem for Linux) to get `sed` working. Alternatively, you could implement similar functionality using Python’s built-in string methods or regex with the re module if you want to keep things pure Python.

Here is another way to modify lines without using `sed`, just in case you want a cross-platform solution:

import re

def replace_in_file_python(input_file, output_file, search_exp, replacement):
    with open(input_file, 'r') as file:
        content = file.readlines()
        
    # Replace lines matching the regex pattern
    with open(output_file, 'w') as out_file:
        for line in content:
            modified_line = re.sub(search_exp, replacement, line)
            out_file.write(modified_line)
    

With this approach, you just run a regex replacement on each line of the file. No need for external commands. Just remember that using regex in Python is a bit different from `sed`, so make sure to check the syntax!

Good luck with your project! If you run into issues, it might be worth checking the exact output or error messages, as they can give clues on what to fix.