# Python style guide#

## Pre commit checks#

Some code-quality checks are performed during continuous integration. The easiest way to check that they pass locally, before submitting your pull request, is by using pre-commit.

Steps to get set up are (run these within your virtual environment):

1. install:

pip install pre-commit

2. enable:

pre-commit install


Now, whenever you stage some file, when you run git commit -m "<some descriptive message>", pre-commit will run the checks defined in .pre-commit-config.yaml and will block your commit if any of them fail. If any hook fails, you should fix it (if necessary), run git add <files> again, and then re-run git commit -m "<some descriptive message>".

You can skip pre-commit using --no-verify, e.g.

git commit -m "wip lol" --no-verify


To skip one particular hook, you can set the SKIP environment variable. E.g. (on Linux):

SKIP=pyupgrade git commit -m "<descriptive message>"


You can manually run all pre-commit hooks on all files with

pre-commit run --all-files


or, if you just want to manually run them on a subset of files,

pre-commit run --files <file_1> <file_2> ... <file_n>


## Gotchas & Troubleshooting#

Pre-commit runs on staged files

If you have some git changes staged and other unstaged, the pre-commit will only run on the staged files.

Pre-commit repeatedly complains about the same formatting changes

Check the unstaged changes (see previous point).

Whitespace changes in the environment-dev.yml files

On Windows, there are some bugs in pre-commit hooks that can lead to changes in some environment YAML files. Until this is fixed upstream, you should ignore these changes. To actually make the commit, deactivate the automated pre-commit with pre-commit uninstall and make sure to run it manually with pre-commit run --all.

Failures in the mypy step

We are running static type checks with mypy to continuously improve the reliability and type safety of the PyMC codebase. However, there are many files with unresolved type problems, which is why we are allowing some files to fail the mypy check.

If you are seeing the mypy step complain, chances are that you are in one of the following two situations:

• 😕 Your changes introduced type problems in a file that was previously free of type problems.

• 🥳 Your changes fixed type problems.

In any case read the logging output of the mypy hook, because it contains the instructions how to proceed.

You can also run the mypy check manually with python scripts/run_mypy.py [--verbose].