Setting up Pants

How to set up Pants for local development.

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Step 1: Fork and clone pantsbuild/pants

We use the popular forking workflow typically used by open source projects. See https://guides.github.com/activities/forking/ for a guide on how to fork pantsbuild/pants, then clone it to your local machine.

macOS users: install a newer openssl

Pants requires a more modern OpenSSL version than the one that comes with macOS. To get all dependencies to resolve correctly, run the below commands. If you are using Zsh, use .zshrc rather than .bashrc.

$ brew install openssl
$ echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.bashrc
$ echo 'export LDFLAGS="-L/usr/local/opt/openssl/lib"' >> ~/.bashrc
$ echo 'export CPPFLAGS="-I/usr/local/opt/openssl/include"' >> ~/.bashrc

(If you don't have brew installed, see https://brew.sh)

Step 2: Set up Git hooks

We use two Git hooks:

1. If you didn't touch any Rust code, we add [ci skip-rust] to your commit message. If you didn't touch any of our release-related code, we add [ci skip-build-wheels] to your commit message.
2. Every time you run git commit, we run some checks and lints.

To install these, run:

$ build-support/bin/setup.sh

You can manually run the pre-commit check by running:

$ build-support/githooks/pre-commit

The Rust-compilation affecting MODE flag is passed through to the hooks, so to run the commit hooks in "debug" mode, you can do something like:

$ MODE=debug git commit ...

How to temporarily skip the pre-commit checks

Use git commit --no-verify or git commit -n to skip the checks.

Step 3: Bootstrap the Rust engine

Pants uses Rustup to install Rust. Run the command from https://rustup.rs to install Rustup; ensure that rustup is on your $PATH.

Then, run ./pants to set up the Python virtual environment and compile the engine.

This will take several minutes

Rust compilation is really slow. Fortunately, this step gets cached, so you will only need to wait the first time.

Want a faster compile?

We default to compiling with Rust's release mode, instead of its debug mode, because this makes Pants substantially faster. However, this results in the compile taking 5-10x longer.

If you are okay with Pants running much slower when iterating, set the environment variable MODE=debug and rerun ./pants to compile in debug mode.

Rust compilation can use lots of storage

Compiling the engine typically results in several gigabytes of storage over time. We have not yet implemented automated garbage collection for building the engine because contributors are the only ones to need to compile Rust, not every-day users.

To free up space, run rm -rf src/rust/engine/target.

Warning: this will cause Rust to recompile everything.

Configure your IDE (optional)

Hooking up the Python virtual environment

Most IDEs allow you to configure a Python virtual environment so that the editor understands your Python import statements.

Pants sets up its development virtualenv at ~/.cache/pants/pants_dev_deps/<arch>.<version>.venv/. Point your editor to the bin/python file in this folder, e.g. ~/.cache/pants/pants_dev_deps/Darwin.py37.venv/bin/python.

PyCharm guide

1. Use "New project" and click the option "Existing interpreter". Point the interpreter to the virtual environment location described above.
2. In your project tree (the list of folders and files), secondary-click the folder src/python. Click "Mark directory as" and choose "Sources".

VSCode guide

Add this to your settings.json file inside the build root's .vscode folder:

settings.json

{
  "python.analysis.extraPaths": ["src/python"],
  "python.formatting.provider": "black",
  "python.linting.enabled": true,
  "python.linting.flake8Enabled": true,
  "python.linting.flake8Args": ["--config=build-support/flake8/.flake8"]
}

python.analysis.extraPaths lets VSCode know where to find Pants's source root. The other config enables black and flake8.