GitHub Actions
Code style: black
Imports: isort

Example Python project that demonstrates how to create a tested Python package using the latest
Python testing and linting tooling. The project contains a fact package that provides a simple
implementation of the factorial algorithm (fact.lib)
and a command line interface (fact.cli).


Python 3.6+.


Because Python 2.7 support ended January 1, 2020, new projects
should consider supporting Python 3 only, which is simpler than trying to support both. As a
result, support for Python 2.7 in this example project has been dropped.

Windows Support

Summary: On Windows, use py instead of python3 for many of the examples in this documentation.

This package fully supports Windows, along with Linux and macOS, but Python is
typically installed differently on Windows. Windows
users typically access Python through the py launcher
rather than a python3 link in their PATH. Within a virtual environment, all platforms operate
the same and use a python link to access the Python version used in that virtual environment.


Dependencies are defined in:

  • requirements.txt
  • dev-requirements.txt

Virtual Environments

It is best practice during development to create an
isolated Python virtual environment using the venv
standard library module. This will keep dependant Python packages from interfering with other
Python projects on your system.

On *Nix:

# On Python 3.9+, add --upgrade-deps
$ python3 -m venv venv
$ source venv/bin/activate

On Windows Powershell / cmd:

> py -m venv venv
> venv\Scripts\activate

Once activated, it is good practice to update core packaging tools (pip, setuptools,
and wheel) to the latest versions.

(venv) $ python -m pip install --upgrade pip setuptools wheel

(Applications Only) Locking Dependencies

This project uses pip-tools to lock project dependencies
and create reproducible virtual environments.

Note: Library projects should not lock their requirements.txt. Since python-blueprint
also has a CLI application, this end-user application example is used to demonstrate how to lock
application dependencies.

To update dependencies:

(venv) $ python -m pip install pip-tools
(venv) $ python -m piptools compile --upgrade
(venv) $ python -m piptools compile --upgrade

After upgrading dependencies, run the unit tests as described in the Unit Testing
section to ensure that none of the updated packages caused incompatibilities in the current

Syncing Virtual Environments

To cleanly install your dependencies into your virtual environment:

(venv) $ python -m piptools sync requirements.txt dev-requirements.txt


This project is designed as a Python package, meaning that it can be bundled up and redistributed
as a single compressed file.

Packaging is configured by:

  • pyproject.toml

To package the project as both a
source distribution and
a wheel:

(venv) $ python sdist bdist_wheel

This will generate dist/fact-1.0.0.tar.gz and dist/fact-1.0.0-py3-none-any.whl.

Read more about the advantages of wheels to understand why generating
wheel distributions are important.

Upload Distributions to PyPI

Source and wheel redistributable packages can
be uploaded to PyPI or installed
directly from the filesystem using pip.

To upload to PyPI:

(venv) $ python -m pip install twine
(venv) $ twine upload dist/*


Automated testing is performed using tox. tox
will automatically create virtual environments based on tox.ini for unit testing, PEP8 style
guide checking, and documentation generation.

# Run all environments.
#   To only run a single environment, specify it like: -e lint
# Note: tox is installed into the virtual environment automatically by ``piptools sync``
# command above.
(venv) $ tox

Unit Testing

Unit testing is performed with pytest. pytest has become the defacto Python
unit testing framework. Some key advantages over the built
in unittest module are:

  1. Significantly less boilerplate needed for tests.
  2. PEP8 compliant names (e.g. pytest.raises() instead of self.assertRaises()).
  3. Vibrant ecosystem of plugins.

pytest will automatically discover and run tests by recursively searching for folders and .py
files prefixed with test for any functions prefixed by test.

The tests folder is created as a Python package (i.e. there is an file within it)
because this helps pytest uniquely namespace the test files. Without this, two test files cannot
be named the same, even if they are in different sub-directories.

Code coverage is provided by the pytest-cov plugin.

When running a unit test tox environment (e.g. tox -e py39), an HTML report is generated in
the htmlcov folder showing each source file and which lines were executed during unit testing.
Open htmlcov/index.html in a web browser to view the report. Code coverage reports help identify
areas of the project that are currently not tested.

Code coverage is configured in pyproject.toml.

To pass arguments to pytest through tox:

(venv) $ tox -e py39 -- -k invalid_factorial

Code Style Checking

PEP8 is the universally accepted style guide for
Python code. PEP8 code compliance is verified using flake8. flake8 is
configured in the [flake8] section of tox.ini. Extra flake8 plugins are also included:

  • pep8-naming: Ensure functions, classes, and variables are named with correct casing.

Automated Code Formatting

Code is automatically formatted using black. Imports are
automatically sorted and grouped using isort.

These tools are configured by:

  • pyproject.toml

To automatically format code, run:

(venv) $ tox -e fmt

To verify code has been formatted, such as in a CI job:

(venv) $ tox -e fmt-check

Generated API Documentation

API Documentation for the fact Python project modules is automatically
generated using a Sphinx tox environment. Sphinx is a documentation
generation tool that is the defacto tool for Python API documentation. Sphinx uses
the RST markup language.

This project uses
the napoleon plugin for
Sphinx, which renders Google-style docstrings. Google-style docstrings provide a good mix of
easy-to-read docstrings in code as well as nicely-rendered output.

"""Computes the factorial through a recursive algorithm.

    n: A positive input value.

    InvalidFactorialError: If n is less than 0.

    Computed factorial.

The Sphinx project is configured in docs/api/

This project uses the furo Sphinx theme for its elegant, simple to
use, dark theme.

Build the docs using the docs-api tox environment (e.g. tox or tox -e docs-api). Once built,
open docs/api/_build/index.html in a web browser.

To configure Sphinx to automatically rebuild when it detects changes, run tox -e docs-api-serve
and open in a browser.

Generate a New Sphinx Project

To generate the Sphinx project shown in this project:

# Note: Sphinx is installed into the virtual environment automatically by ``piptools sync``
# command above.
(venv) $ mkdir -p docs/api
(venv) $ cd docs/api
(venv) $ sphinx-quickstart --no-makefile --no-batchfile --extensions sphinx.ext.napoleon
# When prompted, select all defaults.

Modify appropriately:

# Add the project's Python package to the path so that autodoc can find it.
import os
import sys
sys.path.insert(0, os.path.abspath("../../src"))

Generating a User Guide

Material for MkDocs is a powerful static site
generator that combines easy-to-write Markdown, with a number of Markdown extensions that increase
the power of Markdown. This makes it a great fit for user guides and other technical documentation.

The example MkDocs project included in this project is configured to allow the built documentation
to be hosted at any URL or viewed offline from the file system.

To build the user guide, run tox -e docs-user-guide. Open docs/user_guide/site/index.html using
a web browser.

To build and serve the user guide with automatic rebuilding as you change the contents,
run tox -e docs-user-guide-serve and open in a browser.

Each time the master Git branch is updated, the .github/workflows/pages.yml GitHub Action will
automatically build the user guide and publish it to GitHub Pages.
This is configured in the docs-user-guide-github-pages tox environment. This hosted user guide
can be viewed at

Continuous Integration

Continuous integration is provided by GitHub Actions. This
runs all tests and lints for every commit and pull request to the repository.

GitHub Actions is configured in .github/workflows/python.yml and tox.ini using
the tox-gh-actions plugin.

Project Structure

Traditionally, Python projects place the source for their packages in the root of the project
structure, like:

├── fact
│   ├──
│   ├──
│   └──
├── tests
│   ├──
│   └──
├── tox.ini

However, this structure
is known to
have bad interactions with pytest and tox, two standard tools maintaining Python projects. The
fundamental issue is that tox creates an isolated virtual environment for testing. By installing
the distribution into the virtual environment, tox ensures that the tests pass even after the
distribution has been packaged and installed, thereby catching any errors in packaging and
installation scripts, which are common. Having the Python packages in the project root subverts
this isolation for two reasons:

  1. Calling python in the project root (for example, python -m pytest tests/)
    causes Python to add the current working directory (
    the project root) to sys.path, which Python uses to find modules. Because the source
    package fact is in the project root, it shadows the fact package installed in the tox
  2. Calling pytest directly anywhere that it can find the tests will also add the project root
    to sys.path if the tests folder is a a Python package (that is, it contains a
    pytest adds all folders containing packages
    to sys.path because it imports the tests like regular Python modules.

In order to properly test the project, the source packages must not be on the Python path. To
prevent this, there are three possible solutions:

  1. Remove the file from tests and run pytest directly as a tox command.
  2. Remove the file from tests and change the working directory of python -m pytest
    to tests.
  3. Move the source packages to a dedicated src folder.

The dedicated src directory is the
recommended solution
by pytest when using tox and the solution this blueprint promotes because it is the least brittle
even though it deviates from the traditional Python project structure. It results is a directory
structure like:

├── src
│   └── fact
│       ├──
│       ├──
│       └──
├── tests
│   ├──
│   └──
├── tox.ini

Type Hinting

Type hinting allows developers to include optional
static typing information to Python source code. This allows static analyzers such
as PyCharm, mypy,
or Pyright to check that functions are used with the correct
types before runtime.

For PyCharm in particular,
the IDE is able to provide much richer auto-completion, refactoring, and type checking while the
user types, resulting in increased productivity and correctness.

This project uses the type hinting syntax introduced in Python 3:

def factorial(n: int) -> int:

Type checking is performed by mypy via tox -e type-check. mypy is configured in pyproject.toml.

See also awesome-python-typing.

Distributing Type Hints

PEP 561 defines how a Python package should
communicate the presence of inline type hints to static type
checkers. mypy’s documentation
provides further examples on how to do this as well.

Mypy looks for the existence of a file named py.typed in the root of the installed package to
indicate that inline type hints should be checked.


Licensing for the project is defined in:

  • LICENSE.txt

This project uses a common permissive license, the MIT license.

You may also want to list the licenses of all of the packages that your Python project depends on.
To automatically list the licenses for all dependencies in requirements.txt (and their transitive
dependencies) using pip-licenses:

(venv) $ tox -e licenses
 Name        Version  License
 colorama    0.4.3    BSD License
 exitstatus  1.3.0    MIT License


Docker is a tool that allows for software to be packaged into isolated
containers. It is not necessary to use Docker in a Python project, but for the purposes of
presenting best practice examples, a Docker configuration is provided in this project. The Docker
configuration in this repository is optimized for small size and increased security, rather than

Docker is configured in:

  • Dockerfile
  • .dockerignore

To build the Docker image:

$ docker build --tag fact .

To run the image in a container:

# Example calculating the factorial of 5.
$ docker run --rm --interactive --tty fact 5

PyCharm Configuration

To configure PyCharm 2018.3 and newer to align to the code style used in this project:

  • Settings | Search “Hard wrap at”

    • Editor | Code Style | General | Hard wrap at: 99
  • Settings | Search “Optimize Imports”

    • Editor | Code Style | Python | Imports
      • ☑ Sort import statements
        • ☑ Sort imported names in “from” imports
        • ☐ Sort plain and “from” imports separately within a group
        • ☐ Sort case-insensitively
      • Structure of “from” imports
        • ◎ Leave as is
        • ◉ Join imports with the same source
        • ◎ Always split imports
  • Settings | Search “Docstrings”

    • Tools | Python Integrated Tools | Docstrings | Docstring Format: Google
  • Settings | Search “pytest”

    • Tools | Python Integrated Tools | Testing | Default test runner: pytest
  • Settings | Search “Force parentheses”

    • Editor | Code Style | Python | Wrapping and Braces | “From” Import Statements
      • ☑ Force parentheses if multiline

Integrate Code Formatters

To integrate automatic code formatters into PyCharm, reference the following instructions:

  • black integration

    • The File Watchers method (step 6) is recommended. This will run black on every save.
  • isort integration

    • The File Watchers method (option 1) is recommended. This will run isort on every save.


These tools work best if you properly mark directories as excluded from the project that should
be, such as .tox. See on
how to Right Click | Mark Directory as | Excluded.


View Github