Pre-commit

One of the cornerstones of working with git is remembering to commit your work often. Often committing makes sure that it is easier to identify and revert unwanted changes that you have introduced, because the code changes becomes smaller per commit.

However, as you hopefully already seen in the course there are a lot of mental task to do before you actually write git commit in the terminal. The most basic thing is of course making sure that you have saved all your changes, and you are not committing a not up-to-date file. However, this also includes tasks such as styling, formatting, making sure all tests succeeds etc. All these mental to-do notes does not mix well with the principal of remembering to commit often, because you in principal have to do them every time.

The obvious solution to this problem is to automate all or some of our mental task every time that we do a commit. This is where pre-commit hooks comes into play, as they can help us attach additional tasks that should be run every time that we do a git commit.

Configuration

Pre-commit simply works by inserting whatever workflow we want to automate in between whenever we do a git commit and afterwards would do a git push.

The system works by looking for a file called .pre-commit-config.yaml that we can configure. If we execute

pre-commit sample-config | out-file .pre-commit-config.yaml -encoding utf8

you should get a sample file that looks like

# See https://pre-commit.com for more information
# See https://pre-commit.com/hooks.html for more hooks
repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v3.2.0
    hooks:
    -   id: trailing-whitespace
    -   id: end-of-file-fixer
    -   id: check-yaml
    -   id: check-added-large-files

the file structure is very simple:

It starts by listing the repositories where we want to get our pre-commits from, in this case https://github.com/pre-commit/pre-commit-hooks. This repository contains a large collection of pre-commit hooks.
Next we need to defined what pre-commit hooks that we want to get by specifying the id of the different hooks. The id corresponds to an id in this file: https://github.com/pre-commit/pre-commit-hooks/blob/master/.pre-commit-hooks.yaml

When we are done defining our .pre-commit-config.yaml we just need to install it

pre-commit install

this will make sure that the file is automatically executed whenever we run git commit

❔ Exercises

Install pre-commit
```
pip install pre-commit
```

Next create the sample file

pre-commit sample-config > .pre-commit-config.yaml

The sample file already contains 4 hooks. Make sure you understand what each do and if you need them at all.
pre-commit works by hooking into the git commit command, running whenever that command is run. For this to work, we need to install the hooks into git commit. Run
```
pre-commit install
```
to do this.
Try to commit your recently created .pre-commit-config.yaml file. You will likely not do anything, because pre-commit only check files that are being committed. Instead try to run
```
pre-commit run --all-files
```
that will check every file in your repository.
Try adding at least another check from the base repository to your .pre-commit-config.yaml file.
If you have completed the optional module M7 on good coding practice you will have learned about the linter ruff. ruff comes with its own pre-commit hook. Try adding that to your .pre-commit-config.yaml file and see what happens when you try to commit files.
(Optional) Add more hooks to your .pre-commit-config.yaml.
Sometimes you are in a hurry, so make sure that you also can do commits without running pre-commit e.g.
```
git commit -m <message> --no-verify
```
Finally, figure out how to disable pre-commit again (if you get tired of it).

That was all about how pre-commit can be used to automate tasks. If you want to deep dive more into the topic you can checkout this page on how to define your own pre-commit hooks.