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RustPython Development Guide and Tips

RustPython attracts developers with interest and experience in Rust, Python, or WebAssembly. Whether you are familiar with Rust, Python, or WebAssembly, the goal of this Development Guide is to give you the basics to get set up for developing RustPython and contributing to this project.

The contents of the Development Guide include:

Setting up a development environment

RustPython requires the following:

  • Rust latest stable version (e.g 1.92.0 as of Jan 7 2026)
    • To check Rust version: rustc --version
    • If you have rustup on your system, enter to update to the latest stable version: rustup update stable
    • If you do not have Rust installed, use rustup to do so.
  • CPython version 3.14 or higher
    • CPython can be installed by your operating system's package manager, from the Python website, or using a third-party distribution, such as Anaconda.
  • [macOS] In case of libffi-sys compilation error, make sure autoconf, automake, libtool are installed
    • To install with Homebrew, enter brew install autoconf automake libtool
  • [Optional] The Python package, pytest, is used for testing Python code snippets. To install, enter python3 -m pip install pytest.

Code style

The Rust code style used is the default rustfmt codestyle. Please format your code accordingly, or run cargo fmt to autoformat it. We also use clippy to lint Rust code, which you can check yourself with cargo clippy.

Custom Python code (i.e. code not copied from CPython's standard library) should follow the PEP 8 style. We also use ruff to check Python code style.

In addition to language specific tools, cspell, a code spell checker, is used in order to ensure correct spellings for code.

Testing

To test RustPython's functionality, a collection of Python snippets is located in the extra_tests/snippets directory and can be run using pytest:

$ cd extra_tests
$ pytest -v

Rust unit tests can be run with cargo:

$ cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher

Python unit tests can be run by compiling RustPython and running the test module:

$ cargo run --release -- -m test

There are a few test options that are especially useful:

  • -j <n> enables parallel testing (which is a lot faster), where <n> is the number of threads to be used, ideally the same as number of cores on your CPU. If you don't know, -j 4 or -j 8 are good options.
  • -v enables verbose mode, adding additional information about the tests being run.
  • <test_name> specifies a single test to run instead of running all tests.

For example, to run all tests in parallel:

$ cargo run --release -- -m test -j 4

To run only test_cmath (located at Lib/test/test_cmath) verbosely:

$ cargo run --release -- -m test test_cmath -v

Testing on Linux from macOS

You can test RustPython on Linux from macOS using Apple's container CLI.

Setup (one-time):

# Install container CLI
$ brew install container

# Disable Rosetta requirement for arm64-only builds
$ defaults write com.apple.container.defaults build.rosetta -bool false

# Build the development image
$ container build --arch arm64 -t rustpython-dev -f .devcontainer/Dockerfile .

Running tests:

# Start a persistent container in background (8GB memory, 4 CPUs for compilation)
$ container run -d --name rustpython-test -m 8G -c 4 \
    --mount type=bind,source=$(pwd),target=/workspace \
    -w /workspace rustpython-dev sleep infinity

# Run tests inside the container
$ container exec rustpython-test sh -c "cargo run --release -- -m test test_ensurepip"

# Run any command
$ container exec rustpython-test sh -c "cargo test --workspace"

# Stop and remove the container when done
$ container rm -f rustpython-test

Profiling

To profile RustPython, build it in release mode with the flame-it feature. This will generate a file flamescope.json, which can be viewed at https://speedscope.app.

$ cargo run --release --features flame-it script.py
$ cat flamescope.json
{<json>}

You can specify another file name other than the default by using the --output-file option to specify a file name (or stdout if you specify -). The --output-format option determines the format of the output file. The speedscope json format (default), text, or raw html can be passed. There exists a raw html viewer which is currently broken, and we welcome a PR to fix it.

Code organization

Understanding a new codebase takes time. Here's a brief view of the repository's structure:

  • crates/compiler/src: python compilation to bytecode
    • crates/compiler-core/src: python bytecode representation in rust structures
  • crates/derive/src and crates/derive-impl/src: Rust language extensions and macros specific to rustpython
  • Lib: Carefully selected / copied files from CPython sourcecode. This is the python side of the standard library.
    • test: CPython test suite
  • crates/vm/src: python virtual machine
    • builtins: Builtin functions and types
    • stdlib: Standard library parts implemented in rust.
  • src: using the other subcrates to bring rustpython to life.
  • crates/wasm: Binary crate and resources for WebAssembly build
  • extra_tests: extra integration test snippets as a supplement to Lib/test. Add new RustPython-only regression tests here; do not place new tests under Lib/test.

Understanding Internals

The RustPython workspace includes the rustpython top-level crate. The Cargo.toml file in the root of the repo provide configuration of the crate and the implementation is found in the src directory (specifically, src/lib.rs).

The top-level rustpython binary depends on several lower-level crates including:

  • ruff_python_parser and ruff_python_ast (external dependencies from the Ruff project)
  • rustpython-compiler (implementation in crates/compiler/src)
  • rustpython-vm (implementation in crates/vm/src)

Together, these crates provide the functions of a programming language and enable a line of code to go through a series of steps:

  • parse the line of source code into tokens
  • determine if the tokens are valid syntax
  • create an Abstract Syntax Tree (AST)
  • compile the AST into bytecode
  • execute the bytecode in the virtual machine (VM).

Parser and AST

RustPython uses the Ruff project's parser and AST implementation:

  • Parser: ruff_python_parser is used to convert Python source code into tokens and parse them into an Abstract Syntax Tree (AST)
  • AST: ruff_python_ast provides the Rust types and expressions represented by the AST nodes
  • These are external dependencies maintained by the Ruff project
  • For more information, visit the Ruff GitHub repository

rustpython-compiler

The rustpython-compiler crate's purpose is to transform the AST (Abstract Syntax Tree) to bytecode. The implementation of the compiler is found in the crates/compiler/src directory. The compiler implements Python's symbol table, ast->bytecode compiler, and bytecode optimizer in Rust.

Implementation of bytecode structure in Rust is found in the crates/compiler-core/src directory. crates/compiler-core/src/bytecode.rs contains the representation of instructions and operations in Rust. Further information about Python's bytecode instructions can be found in the Python documentation.

rustpython-vm

The rustpython-vm crate has the important job of running the virtual machine that executes Python's instructions. The crates/vm/src directory contains code to implement the read and evaluation loop that fetches and dispatches instructions. This directory also contains the implementation of the Python Standard Library modules in Rust (crates/vm/src/stdlib). In Python everything can be represented as an object. The crates/vm/src/builtins directory holds the Rust code used to represent different Python objects and their methods. The core implementation of what a Python object is can be found in crates/vm/src/object/core.rs.

Code generation

There are some code generations involved in building RustPython:

  • some part of the AST code is generated from vm/src/stdlib/ast/gen.rs to compiler/ast/src/ast_gen.rs.
  • the __doc__ attributes are generated by the doc project which is then included as the rustpython-doc crate.

Questions

Have you tried these steps and have a question, please chat with us on Discord.