Pre-Commit Hooks

We use pre-commit to manage pre-commit hooks.

Pre-commit hooks are programs which run before each git commit, and can read and potentially modify the files which are to be committed.

We use pre-commit hooks to: - enforce coding guidelines, including the python style-guide PEP8 (black and flake8), - to check the order of import statements (isort), - to check the types of python objects (mypy).

The hooks and their settings are specified in the .pre-commit-config.yaml in each repository.

Handling Pre-Commit Hook Errors

If your git commit fails because of the pre-commit hook, then you should:

1. Run the pre-commit hooks on the files which you have staged, by running the following command in your terminal:

   $ pre-commit run
   

2. Inspect the output. It might look like this:

   $ pre-commit run
   black....................Passed
   isort....................Passed
   flake8...................Passed
   mypy.....................Failed
   - hook id: mypy
   - exit code: 1
   
   example.py:33: error: Need type annotation for "data" (hint: "data: Dict[<type>, <type>] = ...")
   Found 1 errors in 1 files (checked 10 source files)
   

3. Fix any errors which are reported. Important: Once you've changed the code, re-stage the files it to Git. This might mean un-staging changes and then adding them again.
4. If you have trouble:
5. Do a web-search to see if someone else had a similar error in the past.
6. Check that the tests you've written work correctly.
7. Check that there aren't any other obvious errors with the code.
8. If you've done all of that, and you still can't fix the problem, get help from someone else on the team.
9. Repeat 1-4 until all hooks return "passed", e.g.

   $ pre-commit run
   black....................Passed
   isort....................Passed
   flake8...................Passed
   mypy.....................Passed
   

It's easiest to solve these kinds of problems if you make small commits, often.