Writing Git Hooks Using Python

Since git hooks can be any executable script with an appropriate #! line, Python is more than suitable for writing your git hooks. Simply stated, git hooks are scripts which are called at different points of time in the life cycle of working with your git repository.

Let’s start by creating a new git repository:

~/work> git init git-hooks-exp
Initialized empty Git repository in /home/gene/work/git-hooks-exp/.git/
~/work> cd git-hooks-exp/
~/work/git-hooks-exp (master)> tree -al .git/
.git/
├── branches
├── config
├── description
├── HEAD
├── hooks
│   ├── applypatch-msg.sample
│   ├── commit-msg.sample
│   ├── post-update.sample
│   ├── pre-applypatch.sample
│   ├── pre-commit.sample
│   ├── prepare-commit-msg.sample
│   ├── pre-rebase.sample
│   └── update.sample
├── info
│   └── exclude
├── objects
│   ├── info
│   └── pack
└── refs
    ├── heads
    └── tags

9 directories, 12 files

Inside the .git are a number of directories and files, one of them being hooks/ which is where the hooks live. By default, you will have a number of hooks with the file names ending in .sample. They may be useful as starting points for your own scripts. However, since they all have an extension .sample, none of the hooks are actually activated. For a hook to be activated, it must have the right file name and it should be executable. Let’s see how we can write a hook using Python.

We will write a post-commit hook. This hook is called immediately after you have made a commit. We are going to do something fairly useless, but quite interesting in this hook. We will take the commit SHA1 of this commit, and print how it may look like in a more human form. I do the latter using the humanhash module. You will need to have it installed.

Here is how the hook looks like:

#!/usr/bin/python

import subprocess
import humanhash

# get the last commit SHA and print it after humanizing it
# https://github.com/zacharyvoase/humanhash
print humanhash.humanize(
    subprocess.check_output(
        ['git','rev-parse','HEAD']))

I use the subprocess.check_output() function to execute the command git rev-parse HEAD so that I can get the commit SHA1 and then call the humanhash.humanize() function with it.

Save the hook as a file, post-commit in your hooks/ directory and make it executable using chmod +x .git/hooks/post-commit. Let’s see the hook in action:

~/work/git-hooks-exp (master)> touch file
~/work/git-hooks-exp (master)> git add file
~/work/git-hooks-exp (master)> git commit -m "Added a file"
carbon-network-connecticut-equal
[master (root-commit) 2d7880b] Added a file
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 file

The commit SHA1 for the commit turned out to be 2d7880be746a1c1e75844fc1aa161e2b8d955427. Let’s check it with the humanize function and check if we get the same message as above:

>>> humanhash.humanize('2d7880be746a1c1e75844fc1aa161e2b8d955427')
'carbon-network-connecticut-equal'

And you can see the same message above as well. For some of the hooks, you will see that they are called with some parameters. In Python you can access them using the sys.argv attribute from the sys module, with the first member being the name of the hook of course and the others will be the parameters that the hook is called with.