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How to automatically write git commit messages

written by Eric J. Ma on 2023-09-23 | tags: commit messages conventional commits git workflow git llamabot python pre-commit software development data science

(The alternative title to this blog post is actually "How I made LlamaBot able to write commit messages automagically.")

I've finally figured out how to automatically write git commit messages properly within the git workflow!

Commit messages are handy for tracking the history of a project, but it is challenging to write informative ones. There are conventions for writing commits -- one is aptly named Conventional Commits. If you read through it, you'll find it's quite a handful to remember, just like Semantic Versioning. Yet, the conventions exist because they're helpful! If we write commits that follow the Conventional Commits specification, we gain a git history that makes it easy to:

  1. Figure out on which commit a specific change was done (and hence be able to study it or even undo it directly),
  2. Use the collection of commit message headers and bodies to write out accurate and succinct change logs.

For data scientists, the commit messages can be even handier: if we commit one notebook that corresponds to one experiment, then an accurate commit message that summarizes the experiment goals (as described at the top of a notebook) and results (as described at the end of a notebook) essentially lets us have a summarized laboratory notebook.

But here's the kicker: great commit messages take time to write. And nobody has that luxury of time, right? (Unless you're a PhD student!) Nobody can write that level of great commit messages, right?

No longer!

As you might know, I'm hacking on llamabot, which is a Pythonic interface to Large Language Models (LLMs) hosted by OpenAI. One of the things I thought LlamaBot would be able to do is to write commit messages based on the git diff between the current state of the repository and the previous commit. The way this works is that we feed in the git diff output into a LlamaBot SimpleBot and prompt it to write a commit message that follows the Conventional Commits specification.

The Prompt

So, what does the prompt look like?

It's essentially the Conventional Commits specification plus a few more instructions! Here it is taken directly from the source code:

def write_commit_message(diff: str):
    """Please write a commit message for the following diff.

    {{ diff }}

    # noqa: DAR101

    Use the Conventional Commits specification to write the diff.

    <type>[optional scope]: <description>

    [optional body]

    [optional footer(s)]

    The commit contains the following structural elements,
    to communicate intent to the consumers of your library:

    fix: a commit of the type fix patches a bug in your codebase
        (this correlates with PATCH in Semantic Versioning).
    feat: a commit of the type feat introduces a new feature to the codebase
        (this correlates with MINOR in Semantic Versioning).
    BREAKING CHANGE: a commit that has a footer BREAKING CHANGE:,
        or appends a ! after the type/scope,
        introduces a breaking API change
        (correlating with MAJOR in Semantic Versioning).
        A BREAKING CHANGE can be part of commits of any type.

    types other than fix: and feat: are allowed,
    for example @commitlint/config-conventional
    (based on the Angular convention) recommends
    build:, chore:, ci:, docs:, style:, refactor:, perf:, test:, and others.

    footers other than BREAKING CHANGE: <description> may be provided
    and follow a convention similar to git trailer format.

    Additional types are not mandated by the Conventional Commits specification,
    and have no implicit effect in Semantic Versioning
    (unless they include a BREAKING CHANGE).
    A scope may be provided to a commit's type,
    to provide additional contextual information and is contained within parenthesis,
    e.g., feat(parser): add ability to parse arrays.
    Within the optional body section, prefer the use of bullet points.

    Final instructions:

    1. Do not fence the commit message with back-ticks or quotation marks.
    2. Do not add any other text except the commit message itself.
    3. Only write out the commit message.


But that alone doesn't help us much. This prompted bot is magical when it is inserted at precisely the place that's needed: after pre-commit hooks run and before we edit the commit message.

Composing the Commit Message

To compose the commit message, we must first get the diff and feed it into the commit message bot. You can see it below, lifted from the original source:

def compose_commit():
    """Autowrite commit message based on the diff."""
        diff = get_git_diff()
        bot = commitbot()
    except Exception as e:
        echo(f"Error encountered: {e}", err=True)
        echo("Please write your own commit message.", err=True)

As you can see, if there's any error encountered, we still need to enable code committers to write their own manual commit message, so we have a graceful fallback by echoing any errors that show up but not throwing the error. (h/t Andrew Giessel for this design choice.)

But this alone isn't good enough. We must also run the bot after commit hooks and before editing the message.

prepare-commit-msg hook

Turns out, there's a git hook that runs right after the pre-commit hooks and right before editing the message, and it's called the prepare-commit-msg hook. Installing hooks manually is often foreign to data scientists (and I'm guessing a significant fraction of software developers too), so llamabot provides a way of installing the hook, once again lifted from the source:

def install_commit_message_hook():
    """Install a commit message hook that runs the commit message through the bot.

    :raises RuntimeError: If the current directory is not a git repository root.
    # Check that we are in a repository's root. There should be a ".git" folder.
    # Use pathlib to verify.
    if not Path(".git").exists():
        raise RuntimeError(
            "You must be in a git repository root folder to use this command. "
            "Please `cd` into your git repo's root folder and try again, "
            "or use `git init` to create a new repository (if you haven't already)."

    with open(".git/hooks/prepare-commit-msg", "w+") as f:
        contents = """#!/bin/sh
llamabot git compose-commit > .git/COMMIT_EDITMSG
    os.chmod(".git/hooks/prepare-commit-msg", 0o755)
    echo("Commit message hook successfully installed! 🎉")

So to install the hook, one just has to do the following:

llamabot git install-commit-message-hook

This is done once per repository. Note that llamabot must be on the PATH for the hook to work!

How in the world did I come to know this??

git hooks feel like esoteric knowledge. The most commonly known one is the pre-commit hooks. However, the prepare-commit-msg hook is probably not very well-known. As things turned out, I had to ask GPT-4 how to do this a few times with a slightly more refined question each time.

My first question was, "how do I compose a commit message as part of the pre-commit hooks?" which, given my faulty model of git at that point, was the entirely wrong question to ask.

It took me 3-4 tries to get to "how do I compose a commit message using the diff between my current staged changes and the previous commit and use that diff with a SimpleBot program that can automatically write messages to COMMIT_MESSAGE_EDIT" (notice how I even got the filename wrong!). That's when GPT-4 alerted me to the presence of the prepare-msg-hook, which I then read up on the Git book. Only then was my mental model clarified and correct.

I've learned from this that interacting with LLMs requires a certain finesse. GPT-4 can give great code solutions (i.e. "the how") if I can describe with sufficient precision what I'm trying to accomplish (i.e. "the what"). A general description of "the what" doesn't cut it. If you're trying to solve a problem with LLMs, know that possessing precision and clarity in your thinking can go a long way to effectively using LLMs.

Cite this blog post:
    author = {Eric J. Ma},
    title = {How to automatically write git commit messages},
    year = {2023},
    month = {09},
    day = {23},
    howpublished = {\url{}},
    journal = {Eric J. Ma's Blog},
    url = {},

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