Pre-Commit : Customising and Updating

Pre-commit is a tool for running hooks prior to making commits to your Git history. If you’re not familiar with it then you may want to read the earlier post Pre-Commit : Protecting your future self. This article discusses updating pre-commit and is prompted by a change in the flake8 repository.

Pre-commit hooks

A lot of the power of pre-commit comes from the vast array of hooks that are available that users make available. These are included under repos: section of the .pre-commit-config.yaml and typically require a minimum of the repo: and the rev: to use and then optionally a hooks: section. The sample-config that pre-commit will auto-generate looks like…

repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.3.0
    hooks:
    -   id: trailing-whitespace
    -   id: end-of-file-fixer
    -   id: check-yaml
    -   id: check-added-large-files

After finding a repository and hook that you wish to use hooks repository you need to add it to your .pre-commit-config.yaml. Here we add the pylint repository and whilst it only has one hook we explicitly add it.

repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.3.0
    hooks:
    -   id: trailing-whitespace
    -   id: end-of-file-fixer
    -   id: check-yaml
    -   id: check-added-large-files
-   repo: https://github.com/PyCQA/pylint
    rev: v2.15.5
    hooks:
    -   id: pylint

If a repository has more than one hook available then it can be enabled by listing its id: as is the case in the hooks above for the pre-commit-hooks repository.

Local Hooks

In some instances the provisioned repositories do not always meet the requirements. One example of this is the pylint action which parses the code-base to detect errors using pylint. Typically most Python packages have their own dependencies but because the Pylint action pulls down and uses its own virtual environment these packages are not installed. As a consequence pylint reports a lot of import-error as its unable to import the required dependencies.

The solution to this is to write a local hook, which instead of defining a GitHub repository as the repo: uses the value local. Thus to run pylint in a local environment from pre-commit you would add the following to your .pre-commit-config.yaml

repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.3.0
    hooks:
    -   id: trailing-whitespace
    -   id: end-of-file-fixer
    -   id: check-yaml
    -   id: check-added-large-files
# -   repo: https://github.com/PyCQA/pylint
#     rev: v2.15.5
#     hooks:
#     -   id: pylint
-   repo: local
    hooks:
    -   id: pylint
        name: PyLint
        entry: python -m pylint.__main__
        language: system
        files: \.py$

For this to work you would have to ensure that you have a virtual environment activated that includes the package dependencies, including pylint, when you make you git commit so that pre-commit can find and import all the required packages.

Updating pre-commit

After adding a new repo and hook it will not be immediately ready to use as the environment has not been initialised. You can wait until your next commit or force this with the autoupdate option. This will update all repositories that are defined in your configuration.

$ pre-commit autoupdate
Updating https://github.com/pre-commit/pre-commit-hooks ... updating v3.2.0 -> v4.3.0.
Updating https://github.com/PyCQA/pylint ... [INFO] Initializing environment for https://github.com/PyCQA/pylint.
already up to date.

Repository Changes

Sometimes, albeit rarely, repositories change their location as was the case recently when flake8 moved from GitLab to GitHub. As a consequence any pre-commit that uses flake8 repo/hook and configured to run in Continuous Integration pipelines failed as it was unable to download and run the flake8 environment. The solution is simply to update the repo:.

Before this change the entry for flake8 looked like…

-   repo: https://gitlab.com/pycqa/flake8.git
    rev: 3.9.2
    hooks:
    -   id: flake8
        additional_dependencies: [flake8-print]
        args: ["topostats", "tests"]
        types: [python]

To update to use the new repository it should point to github.com as shown below.

-   repo: https://github.com/pycqa/flake8.git
    rev: 3.9.2
    hooks:
    -   id: flake8
        additional_dependencies: [flake8-print]
        args: ["topostats", "tests"]
        types: [python]

After making this change you have to pre-commit autoupdate to force downloading and updating from the new source, otherwise your existing older revision will be used locally.

Links

• Pre-Commit : Protecting your future self
• Pre-commit
• Pre-commit hooks
• pylint
• flake8