How to set up a perfect Python project


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When starting a new Python project, it is tempting to just dive in and start coding. Spending a tiny amount of time to setup a project with the best tools will save immense time and lead to a happier coding experience.

In an ideal world, dependencies would be identical for all developers, code would be perfectly formatted, common errors forbidden and everything would be covered by tests. Additionally, all of these would ensured at each commit.

In this article I'll go through how to set up a project that does exactly that. You can either follow along with the steps or jump straight to generating a new project automatically by installing pipx and pipenv then generating a new project.

Let's create a new project directory:

mkdir best_practices
cd best_practices

Python command line tools with pipx

Pipx is a handy utility that allows quick installation of python command line tools. We'll be using it to install pipenv and cookiecutter.

python3 -m pip install --user pipx
python3 -m pipx ensurepath

Dependency management with pipenv

Pipenv automatically creates and manages a virtualenv for your projects, as well as adding/removing packages from your Pipfile as you install/uninstall packages. It also generates the ever-important Pipfile.lock, which is used to produce deterministic builds.

Knowing that you and your teammates are using the same library versions is a huge confidence boost. Pipenv takes care of this and has gained massive traction in the last year or so.

pipx install pipenv

Code formatting with black and isort

Black formats our code:

Black is the uncompromising Python code formatter. By using it, you agree to cede control over minutiae of hand-formatting. In return, Black gives you speed, determinism, and freedom from pycodestyle nagging about formatting. You will save time and mental energy for more important matters.

Blackened code looks the same regardless of the project you're reading. Formatting becomes transparent after a while and you can focus on the content instead.

Black makes code review faster by producing the smallest diffs possible.

while isort sorts our imports:

isort your python imports for you so you don't have to. isort is a Python utility / library to sort imports alphabetically, and automatically separated into sections.

Let's install them using pipenv as development dependencies so they don't clutter a deployment:

pipenv install black isort --dev

Black and isort have incompatible default options so we'll override isort to follow black's lead. Create a setup.cfg file and add this config:

[isort]
multi_line_output=3
include_trailing_comma=True
force_grid_wrap=0
use_parentheses=True
line_length=88

We can run these tools with:

pipenv run black
pipenv run isort

Style enforcement with flake8

Flake8 ensures our code follows the standard python conventions as defined in PEP8. Install using pipenv:

pipenv install flake8 --dev

Just like isort it needs a little configuration to work well with black. Add this config to setup.cfg:

[flake8]
ignore = E203, E266, E501, W503
max-line-length = 88
max-complexity = 18
select = B,C,E,F,W,T4

Now we can run flake8 with pipenv run flake8.

Static types with mypy

Mypy is an optional static type checker for Python that aims to combine the benefits of dynamic (or "duck") typing and static typing. Mypy combines the expressive power and convenience of Python with a powerful type system and compile-time type checking. Mypy type checks standard Python programs; run them using any Python VM with basically no runtime overhead.

Having types in Python takes a little getting used to but the benefits are substantial. From the website:

  • Static typing can make programs easier to understand and maintain
  • Static typing can help you find bugs earlier and with less testing and debugging
  • Static typing can help you find difficult-to-find bugs before your code goes into production
pipenv install mypy --dev

Mypy by default will recursively check all imports for type annotations which leads to errors when libraries do not include these annotations. We need to configure mypy to run only on our code and to ignore any errors for imports without type annotations. We are assuming that our code lives in the best_practices package for the following config. Add this to setup.cfg:

[mypy]
files=best_practices,test
ignore_missing_imports=true

Now we can run mypy with:

pipenv run mypy

Here's a useful cheat sheet for using it.

Testing with pytest and pytest-cov

Writing tests with pytest is incredibly easy and removing any friction to writing tests means we will write more of them!

pipenv install pytest pytest-cov --dev

Here's a simple example from the pytest website:

# content of test_sample.py
def inc(x):
    return x + 1


def test_answer():
    assert inc(3) == 5

To execute it:

$ pipenv run pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
cachedir: $PYTHON_PREFIX/.pytest_cache
rootdir: $REGENDOC_TMPDIR
collected 1 item

test_sample.py F                                                     [100%]

================================= FAILURES =================================
_______________________________ test_answer ________________________________

    def test_answer():
>       assert inc(3) == 5
E       assert 4 == 5
E        +  where 4 = inc(3)

test_sample.py:6: AssertionError
========================= 1 failed in 0.12 seconds =========================

All our tests should go in the test directory so add this config to setup.cfg:

[tool:pytest]
testpaths=test

We also want to check how much of our code is covered by tests. Create a new file .coveragerc to only return coverage statistics for our application code, again we are assuming our application code lives in the best_practices module:

[run]
source = best_practices

[report]
exclude_lines =
    # Have to re-enable the standard pragma
    pragma: no cover

    # Don't complain about missing debug-only code:
    def __repr__
    if self\.debug

    # Don't complain if tests don't hit defensive assertion code:
    raise AssertionError
    raise NotImplementedError

    # Don't complain if non-runnable code isn't run:
    if 0:
    if __name__ == .__main__.:

We can now run our tests and report coverage with

pipenv run pytest --cov --cov-fail-under=100

This will fail if our test coverage of the application code is less than 100%.

Git hooks with pre-commit

Git hooks allow you to run scripts any time you want to commit or push. This lets us run all of our linting and tests automatically every time we commit/push. pre‑commit allows easy configuration of these hooks:

Git hook scripts are useful for identifying simple issues before submission to code review. We run our hooks on every commit to automatically point out issues in code such as missing semicolons, trailing whitespace, and debug statements. By pointing these issues out before code review, this allows a code reviewer to focus on the architecture of a change while not wasting time with trivial style nitpicks.

Here we configure all of the above tools to run on any changed python files on committing, and also to run pytest coverage only when pushing as it can be slow. Create a new file .pre-commit-config.yaml:

repos:
- repo: local
  hooks:
  - id: isort
    name: isort
    stages: [commit]
    language: system
    entry: pipenv run isort
    types: [python]

  - id: black
    name: black
    stages: [commit]
    language: system
    entry: pipenv run black
    types: [python]

  - id: flake8
    name: flake8
    stages: [commit]
    language: system
    entry: pipenv run flake8
    types: [python]
    exclude: setup.py

  - id: mypy
    name: mypy
    stages: [commit]
    language: system
    entry: pipenv run mypy
    types: [python]
    pass_filenames: false

  - id: pytest
    name: pytest
    stages: [commit]
    language: system
    entry: pipenv run pytest
    types: [python]

  - id: pytest-cov
    name: pytest
    stages: [push]
    language: system
    entry: pipenv run pytest --cov --cov-fail-under=100
    types: [python]
    pass_filenames: false

If you ever need to skip these hooks you can run git commit --no-verify or git push --no-verify

Generate a project using cookiecutter

Now we've seen what an ideal project contains, we can turn this into a template to generate a new project with a single command:

pipx run cookiecutter gh:sourceryai/python-best-practices-cookiecutter

Fill in the project name and repo name and your project will be generated for you.

To finish setting up, follow these steps:

# Enter project directory
cd <repo_name>

# Initialise git repo
git init

# Install dependencies
pipenv install --dev

# Setup pre-commit and pre-push hooks
pipenv run pre-commit install -t pre-commit
pipenv run pre-commit install -t pre-push

The template project contains a very simple Python file and test to try the tools out on. Once you're happy with the code you can then do your first git commit and all the hooks will be run.

Editor Integration

While it is great to know that the standard of code on our project will always be maintained at the highest level when committing code, it is somewhat frustrating to find out any issues after we think the code changes have all been done. It is much better to get the issues displayed in real-time.

Spend some time to make sure these commands are run by your code editor when saving a file. Having instant feedback means you can quickly fixup any minor issues introduced while the code is fresh in the mind.

Personally I use some excellent Vim plugins to accomplish this:

Sourcery

The next step is to have your editor automatically refactor your code for you as you work. This is why we built Sourcery, give it a go!