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Python Polylith Example

This is a repository with an example Python setup of the Polylith Architecture using pants. Here you will find examples of code being shared between different kind of projects, and the developer tooling setup.

Have a look at this repository for more information and documentation: Python tools for the Polylith Architecture

Pants and Polylith

A big part of the Developer Experience with Polylith is having access to all of the code in the Monorepo, including the third-party dependencies. Having access to Python code from a REPL and an IDE would require an existing virtual environment.

Pants has really good support for creating virtual environments.

In the Python Community, there is a convention to name the environment in a certain way, usually .venv, and create it at the Project root (this will play nice with your IDE). The virtual environment created by Pants is created in a dists folder, and further in a Pants-specific folder structure.

In addition to the .venv, package management tools like Poetry, Hatch and PDM have support for configuring custom source paths. This is useful for the IDE to locate Python source code (especially in Monorepos).

This is usually accomplished by creating a .pth file, that is stored in the site_packages folder of the virtual environment. Tools like Poetry, Hatch and PDM do that when the virtual environment is created.

With a .pth file in place, you will be able to navigate the Python source code from the IDE as expected.

A custom bash script

At the development/scripts folder of this repo, you will find a generate-venv.sh that will create a .venv with a .pth file. You are now all set and ready to develop!

The Polylith CLI

By adding the polylith-cli library to the top pyproject.toml, the poly command will be available in the virtual environment.

Activate the virtual environment:

source .venv/bin/activate

You have now access to all the poly commands.

Example:

poly info

The REPL

With the virtual environment activated, you can also run code using your favorite REPL:

ipython

Packaging projects

Packaging a project can be done using Pants.

Example: creating a wheel and an sdist for the my-fastapi-project

pants package projects/my_fastapi_project:my-fastapi-project

PEP 517

Pants will expect a BUILD file in the project folder. The pants package goal has support for the PEP 517 specification.

Polylith is using the pyproject.toml to define the used bricks in a project. By adding Hatchling as the build backend, Pants will act as the build frontend for it and the artifacts will be created according to what's defined in the pyproject.toml.

The BUILD file of a project: add the pyproject.toml and the entry point as dependencies.

resource(name="pyproject", source="pyproject.toml")

python_distribution(
    name="my-fastapi-project",
    dependencies=[
        ":pyproject",
        "bases/example/greet_api:greet_api",
    ],
    provides=python_artifact(),
    generate_setup = False,
)

A project-specific configuration

Example, the build backend:

[build-system]
requires = ["hatchling", "hatch-polylith-bricks"]
build-backend = "hatchling.build"

Example, the Polylith bricks:

[tool.hatch.build.hooks.polylith-bricks]

[tool.polylith.bricks]
"../../bases/example/greet_api" = "example/greet_api"
"../../components/example/greeting" = "example/greeting"
"../../components/example/log" = "example/log"