hook.py

#!/usr/bin/env python

# NOTE: Put it as your global prepare-commit-msg hook.
#       Usually at `~/.git-templates/hooks/prepare-commit-msg`.
#       REMEMBER to make it executable with `chmod +x ~/.git-templates/hooks/prepare-commit-msg`.
#
#       After that all new repositories should have this hook installed.
#       In existing repos you need to run `git init` to apply this hook.

import re
import sys
from subprocess import CalledProcessError, check_output


def get_branch_name():
    try:
        return (
            check_output(["git", "symbolic-ref", "--short", "HEAD"])
            .strip()
            .decode("utf-8")
        )
    except CalledProcessError:
        sys.exit(0)


def get_commit_msg(commit_msg_filepath):
    return open(commit_msg_filepath).read()


def get_commit_msg_with_item_number(branch_name, commit_msg):
    regex = r"(\d+)[\-\_]"

    if not re.findall(regex, branch_name):
        return commit_msg
    try:
        item_number = re.findall(regex, branch_name)[0]
    except IndexError:
        return commit_msg

    item_text = f"Related item: #{item_number}"

    text_to_add = f"\n\n{item_text}\n\n"

    if item_text in commit_msg:
        return commit_msg

    splitter = "# "

    try:
        first_part, second_part = commit_msg.split(splitter, maxsplit=1)

        second_part = splitter + second_part
        return first_part.strip() + text_to_add + second_part.strip()
    except ValueError:
        pass

    return f"{commit_msg}{text_to_add}"


def save_commit_msg(new_commit_msg, commit_msg_filepath):
    with open(commit_msg_filepath, "r+") as f:
        f.seek(0, 0)
        f.write(new_commit_msg)


if __name__ == "__main__":
    branch_name = get_branch_name()

    commit_msg_filepath = sys.argv[1]
    commit_msg = get_commit_msg(commit_msg_filepath)

    new_commit_msg = get_commit_msg_with_item_number(
        branch_name=branch_name, commit_msg=commit_msg
    )
    if new_commit_msg:
        save_commit_msg(
            new_commit_msg=new_commit_msg, commit_msg_filepath=commit_msg_filepath
        )

prepare-commit-msg.gif

Fixed few things. Now it will work also if you commit using fx. VSCode