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

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

Step 2: Bootstrap the Rust engine

Pants requires several dependencies to be installed: a Python 3.9 interpreter, Rust, the protobuf compiler, clang and others. There is experimental support for the Nix package manager that makes it easy to set up a dev environment. Follow the instructions on the Nix website to install Nix. Then cd into the directory where you cloned the Pants repo and type nix-shell. This will download all the necessary dependencies and start a shell with a suitably configured PATH variable to make them available for use.

Alternatively, you can install the dependencies manually as follows:

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

If your system Python is not the version Pants expects (currently Python 3.9), you'll need to provide one. Python interpreters from Linux or Mac distributions sometimes have quirks that can cause headaches with bootstrapping the dev venv. Some examples of Pythons that work well with Pants are those provided by:

• Fedora
• ASDF
• PyEnv Providers that sometimes cause issues include:
• Ubuntu Deadsnakes You also need to have the protobuf compiler and LLVM clang installed. On Debian derivatives, these can be installed using apt install clang protobuf-compiler.

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.

Step 3: Set up a pre-push Git Hook

We have a Git Hook that runs some useful checks and lints when you git push. Running this locally can prevent easily avoidable CI failures such as whitespace or linting issues.

To install this, run:

$ build-support/bin/setup.sh

You can manually run the pre-push check with:

$ build-support/githooks/pre-push

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

$ MODE=debug git push ...

How to temporarily skip the pre-push checks

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

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"],
  "rust-analyzer.linkedProjects": ["src/rust/engine/Cargo.toml"]
}

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