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Style Guide


Scikit-HEP uses pre-commit to check code style. It can be installed through brew (macOS) or pip (anywhere). There are two modes to use it locally; you can check manually with pre-commit run (changes only) or pre-commit run --all-files (all). You can also run pre-commit install to add checks as a git precommit hook (which is where it gets its name). It’s worth trying, even if you’ve tried and failed to set up a custom precommit hook before; it’s quite elegant and does not add or commit the changes, it just makes the changes and allows you to check and add them. You can always override the hook with -n.

Here is a minimal .pre-commit-config.yaml file with some handy options:

- repo:
  rev: v3.3.0
  - id: check-added-large-files
  - id: check-case-conflict
  - id: check-merge-conflict
  - id: check-symlinks
  - id: check-yaml
  - id: debug-statements
  - id: end-of-file-fixer
  - id: mixed-line-ending
  - id: requirements-txt-fixer
  - id: trailing-whitespace

For a Python 2+3 codebase, the following is also useful:

  - id: fix-encoding-pragma

Helpful tip: Pre-commit runs top-to-bottom, so put checks that modify content (like the several of the pre-commit-hooks above, or Black) above checks that might be more likely to pass after the modification (like flake8).

Keeping pinned versions fresh: You can use pre-commit autoupdate to move your tagged versions forward to the latest tags!


Black is a popular auto-formatter from the Python Software Foundation. One of the main features of Black is that it is “opinionated”; that is, it is almost completely unconfigurable. Instead of allowing you to come up with your own format, it enforces one on you. While I am quite sure you can come up with a better format, having a single standard makes it possible to learn to read code very fast - you can immediately see nested lists, matching brackets, etc. There also is a faction of developers that dislikes all auto-formatting tools, but inside a system like pre-commit, auto-formatters are ideal. They also speed up the writing of code because you can ignore formatting your code when you write it. By imposing a standard, all Scikit-HEP developers can quickly read any package’s code.

Also, properly formatted code has other benefits, such as if two developers make the same change, they get the same formatting, and merge requests are easier. The style choices in Black were explicitly made to optimize git diffs!

There are a few options, mostly to enable/disable certain files and to change the line length, and those go in your pyproject.toml file.

Here is the snippet to add Black to your .pre-commit-config.yml:

- repo:
  rev: 20.08b1
  - id: black

You can add a Black badge to your repo as well if you want.

[![Code style: black](](
.. image::

In very specific situations, you may want to retain special formatting. After carefully deciding that it is a special use case, you can use # fmt: on and #fmt: off around a code block to have it keep custom formatting. Always consider refactoring before you try this option! Most of the time, you can find a way to make the Blacked code look better by rewriting your code; factor out long unreadable portions into a variable, avoid writing matrices as 1D lists, etc.


Check-manifest is a fantastic, highly recommended tool that verifies you have working SDists. You can install it from PyPI. Run it on your repository and see what it says. If you want to ignore files (like test folders, example folders, docs, etc) you can add these into your config files, either pyproject.toml or setup.cfg:

# pyproject.toml
ignore = [".travis.yml"]

# setup.cfg or tox.ini
ignore =

Add the following to your pre-commit config:

- repo:
  rev: "0.45"
  - id: check-manifest

If you use setuptools_scm, you might want to add:

    additional-dependencies: [setuptools_scm, toml]
If this is too slow: (click here)

Warning: For a complex package, this may be slow. You can optionally set stages: [manual] just below the id, and then only run this explicitly (probably in CI only). In GHA, you should enable the manual stage, which will run all checks:

    - uses: pre-commit/action@v2.0.0
        extra_args: --all-files --hook-stage manual

Type checking (new)

One of the most exciting advancements in Python in the last 10 years has been static type hints. Scikit-HEP is just beginning to make sure packages are type-hint ready. One of the challenges for providing static type hints is that it was developed in the Python 3 era and it really shines in a Python 3.7+ codebase (due to from __future__ import annotations, which turns annotations into strings and allows you to use future Python features in Python 3.7+ annotations as long as your type checker supports them). For now, it is recommended that you make an attempt to support type checking through your public API in the best way that you can (based on your supported Python versions). Stub files or type comments allow Python 2 or Python 3.5 to be supported. MyPy is suggested for type checking, though there are several other good options to try, as well. If you have built-in support for type checking, you need to add empty py.typed files to all packages/subpackages to indicate that you support it.

The MyPy addition for pre-commit:

- repo:
  rev: v0.790
  - id: mypy
    files: src

You can also add items to the virtual environment setup for mypy by pre-commit, for example:

    additional_dependencies: [attrs==19.3.0]

MyPy has a config section in setup.cfg that looks like this:

files = src
pretty = True
python_version = 3.6
warn_unused_configs = True
warn_unused_ignores = True

ignore_missing_imports = True

There are a lot of options, and you can start with only typing global code and functions with at least one type annotation (the default) and enable more checks as you go. You can ignore missing imports on libraries as shown above, on section each. And you can disable MyPy on a line with # type: ignore. Once you are ready to start checking more, you can look at adding check_untyped_defs, disallow_untyped_defs, disallow_incomplete_defs, and more, up until strict. You can add these per file by adding a # mypy: strict (or any other less stringent check) at the top of the file.


Flake8 can check a collection of good practices for you, ranging from simple style to things that might confuse or detract users, such as unused imports, named values that are never used, mutable default arguments, and more. Unlike black and some other tools, flake8 does not correct problems, it just reports them. Some of the checks could have had automated fixes, sadly (which is why Black is nice). Here is a suggested setup.cfg to enable compatibility with Black:

ignore = E203, E231, E501, W503
select = C,E,F,W

One recommended plugin for flake8 is flake8-bugbear, which catches many common bugs. It is highly opinionated and can be made more so with the B9 setting. You can also set a max complexity, which bugs you when you have complex functions that should be broken up. Here is an opinionated config:

max-complexity = 12
ignore = E203, E231, E501, E722, W503
select = C,E,F,W,B,B9

(Error E722 is identical to B001.) Here is the flake8 addition for pre-commit, with the bugbear plugin:

- repo:
  rev: 3.8.4
  - id: flake8
    additional_dependencies: [flake8-bugbear]

This will be too much at first, so you can disable or enable any test by it’s label. You can also disable a check or a list of checks inline with # noqa: X### (where you list the check label(s)). Over time, you can fix and enable more checks. A few interesting plugins:

  • flake8-bugbear: Fantastic checker that catches common situations that tend to create bugs. Codes: B, B9
  • flake8-docstrings: Docstring checker. --docstring-convention=pep257 is default, numpy and google also allowed.
  • flake8-spellcheck: Spelling checker. Code: SC
  • flake8-import-order: Enforces PEP8 grouped imports (which are quite nice). Code: I
  • pep8-naming: Enforces PEP8 naming rules. Code: N
  • flake8-print: Makes sure you don’t have print statements that sneak in. Code: T
Flake8-print details: (click here)

Having something verify you don’t add a print statement by mistake is very useful. A common need for the print checker would be to add it to a single directory (src if you are following the convention recommended). You can do the next best thing by removing directories and file just for this check (T) in your flake8 config:

select = C,E,F,W,T
per-file-ignores =
    tests/*: T
    examples/*: T

Python warnings

Python hides important warnings by default, mostly because it’s trying to be nice to users. You are a developer, you don’t want it to be “nice”. You want to find and fix warnings before they cause user errors! Always either run Python/PyTest 3.1+ with -Wd, or set export PYTHONWARNINGS=d in your environment. You can also add the following to your setup.cfg file:

addopts = -Wd

Clang-format (C++ only)

If you have C++ code, you should have a .clang-format file and use the following pre-commit config:

- repo: local
  - id: docker-clang-format
    name: Docker Clang Format
    language: docker_image
    - c++
    entry: unibeautify/clang-format:latest
    - -style=file
    - -i
For a non-docker version: (click here)

You can make a similar non-docker file, but that should sit beside the docker one for use on CI. You can install this non-docker file locally if you want it and have clang already installed. Note that formatting changes between versions, so only the above recipe is guaranteed to work identically to CI. The local one would look like this:

- repo: local
  - id: clang-format
    name: Clang Format
    language: system
    - c++
    entry: clang-format
    - -style=file
    - -i