Precommit

Better Git Hooks

Git hooks are scripts that are trigered when some action from Git is perfomed. They can run before or after common actions such creating a commit, push or pull.

This can be used to define a local quality gate for the code that is triggered before each commit, push or pull action. Therefore, they enable catching errors ahead and preventing defective code been commited or sended to the remote server.

One can create any hook by adding a executable script at the .git/hooks stored in the repository home. But there is a more pratical for managing and maintaining Git hooks by using the Python package Pre-commit.

It uses a yaml config file placed in the repository as source, install and run all hooks.

Installing Pre-commit

It can be installed using pip using the following command:

pip install pre-commit

Basic Configuration

Once pre-commit is installed, you can create a file .pre-commit-config.yaml in the repository root with the following content:

repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.4.0
    hooks:
    -   id: end-of-file-fixer
    -   id: check-yaml
    -   id: check-added-large-files
    -   id: check-merge-conflict

The hooks end-of-file-fixer, check-yaml, check-added-large-files, check-merge-conflict are already defined inside the the repository https://github.com/pre-commit/pre-commit-hooks.

In order to install them, just run the following command

pre-commit install -c .pre-commit-config.yaml

This will download the repository and install the hooks inside the .git/hooks folder. Then, the pre-commit hooks registered will run everytime that you run git commit before the commit be generated.

Using hooks with Parameters

pre-commit enable also to use some tools

For instance, if you would like to use the following command with all python files inside the a repository:

yapf --style=pep8 --parallel --in-place

This can be translated to the following config lines:

repos:
# ...
-   repo: https://github.com/google/yapf                     # yapf repository url
    rev: v0.40.0                                             # repository version to be used
    hooks:
    -   id: yapf                                             # id (used to call hook individualy)
        name: Ensure PEP-8 complience for all Python files   # short description
        args:                                                # Define arguments to be passed whenever yapf is called 
          - '--style=pep8'                                   # yapf arg: Ensure PEP-8
          - '--parallel'                                     # yapf arg: Allow run in parallel
          - '--in-place'                                     # yapf arg: edit file and fix error found
        files: \.(py)$                                       # Apply hook to all *.py files and trigger hook whenever a python file is changed

Using local commands as custom hooks

Some tools might be available only localy. In this case, you can use the repo: local combined with language: system to translate the command required by the tool to hook in the yaml config file.

For instance, the following command

pytest --junitxml=pytest.xml --cov-report=xml --cov=src -v

Can be translated to

repos:
# ...
-   repo: local
    hooks:
    -   id: pytest
        name: Ensure all tests are passing
        entry: pytest
        language: system                    # Uses entry as direct system command
        args:
          - '--junitxml=pytest.xml'
          - '--cov-report=xml'
          - '--cov=src'
          - '-v'
        files: \.(py)$                      # Trigger hook whenever a python file is changed
        always_run: true                    # This will the command will always run
        pass_filenames: false               # This will prevent the files to be passed to command since some tools might not need.

Git++

There so much more! Go ahead check the documentation page at https://pre-commit.com/ to enable even more powerful hooks.