merge two csv files into output.csv using Subprocess

[DeaD_EyE]

Use Python instead of depending on OS infrastructure. This program could not run on Linux/Mac for example.
Here an example function to merge files:

from pathlib import Path


def merge_files(input_files: list[str | Path], output_file: str | Path) -> None:
    # user of code could supply a list with paths as str
    # or the paths as Path objetcs
    # but later Path objects are required, so all objects
    # are converted here
    input_files = [Path(file) for file in input_files]
    output_file = Path(output_file)
    
    # opening file in binary mode, which prevents encoding errors
    # if all input_files are encoded with the same encoding
    # Path objects do have the open method, which supports context managers
    with output_file.open("wb") as fd_out:
        # iterating over all input files
        for input_file in input_files:
            # same here, opening the input file in binary mode
            with input_file.open("rb") as fd_in:
                # https://realpython.com/lessons/assignment-expressions/
                # read 4KiB chunks
                while chunk := fd_in.read(4 * 1024 ** 1):
                    fd_out.write(chunk)
                
                # if the last line of input_file has no
                # lineseperator at the end, adding one to it
                # keep in mind, that everything is in binary mode
                
                # os.linesep is OS depended
                if not chunk.endswith(os.linesep.encode("ascii")):
                    fd_out.write(os.linesep.encode("ascii"))