prefix commit messages with branch names

prepare-commit-msg

#!/usr/bin/python
"""
This is a prepare-commit-msg hook that fill prefix commit messages with
[BRANCH]

Copy this file to $GITREPOSITORY/.git/hooks/prepare-commit-msg
and mark it executable.

Assumes your branch is based off of a tracker ID
"""
import subprocess
import sys
import os


def which(executable):
    locations = (
        '/usr/local/bin',
        '/bin',
        '/usr/bin',
        '/usr/local/sbin',
        '/usr/sbin',
        '/sbin',
    )

    for location in locations:
        executable_path = os.path.join(location, executable)
        if os.path.exists(executable_path):
            return executable_path

GIT = which('git')


def branch_name():
    try:
        name = subprocess.check_output(
            [GIT, "symbolic-ref", "HEAD"],
            stderr=subprocess.STDOUT)
    except Exception as err:
        if 'fatal: ref HEAD is not a symbolic ref' in err.output:
            # we are in a rebase or detached head state
            return ''
    # This looks like: refs/heads/12345678/my-cool-feature
    # if we ever get a branch that has '/' in it we are going to have
    # some issues.
    return name.split('/')[-1]
    parts = name.split('/')
    if len(parts) != 4:
        raise ValueError("Branch name has '/' in it which is not allowed")
    branch = parts[-1]
    return branch


def prepend_commit_msg(branch):
    """Prepend the commit message with `text`"""
    msgfile = sys.argv[1]
    header = "[%s] " % branch

    with open(msgfile) as f:
        contents = f.read()

    if not branch:
        return contents

    with open(msgfile, 'w') as f:
        # Don't append if it's already there
        if not contents.startswith(header):
            f.write(header)
        f.write(contents)


def main():
    branch = branch_name().strip().strip('\n')
    prepend_commit_msg(branch)

if __name__ == '__main__':
    main()