Local Development#

Here we’ll describe how to get a reproducible local development environment setup.

Environment Tools#

Install Rustup#

Install a Rust compiler stack. See the Rust Project’s installation documentation on how to do that using rustup.

You don’t have to actually install a specific version of the toolchain, just install rustup. The build process below will ensure you use the correct version.

Install just#

We use just as a command runner for actions / recipes related to developing Bytewax. Please follow the installation instructions. There’s probably a package for your OS already.

Install pyenv and Python 3.12#

We want Bytewax to run under many different versions of the Python interpreter, so we need a quick way to create virtualenvs in different versions. I suggest using pyenv because it will let you re-compile Python easily with benchmarking flags. Follow the installation instructions.

You can also use your OS’s package manager to get access to different Python versions.

Our blessed version of Python for doing development is Python 3.12 (although the Bytewax library is designed to run on versions 3.8+).

Ensure that you have Python 3.12 installed and available as a “global shim” so that it can be run anywhere. The following will make plain python run your OS-wide interpreter, but will make 3.12 available via python3.12.

$ pyenv global system 3.12

Install uv#

We use uv as a virtual environment creator, package installer, and dependency pin-er. There are a few different ways to install it, but I recommend installing it through either brew on macOS or pipx.

Setup the Reproducible Development Virtual Environment#

We have a “getting started” just recipe that will:

  1. Set up a venv in venvs/dev/.

  2. Install all development dependencies into it in a reproducible way.

  3. Install Git pre-commit hooks that use that venv.

$ just get-started

If it detects anything wrong, follow the instructions.

Whenever you work on Bytewax, you must have the dev venv activated. Most of the other just recipes require this. They will error if the venv is not activated.


Prompts here will be prefixed with (dev) to show that the venv is active.

$ . venvs/dev/bin/activate
(dev) $

LSP and Editors#

Since all the development tools are installed into the dev venv, find a way to point VSCode, PyCharm, or whatever IDE / LSP you are using at the venv located in the venvs/dev/ directory. It is important you do this because pyproject.toml has settings for how the LSP should handle various cases and will format or lint or give type hints incorrectly if a different environment is used.

Common Actions#

Most of our common actions for developing on the library are just recipes.

Build Library#

This will compile the Rust code for the Bytewax library, generate new stubs, and install it into the current venv. You can then use ipython, or run a dataflow, or run unit tests to try it out.

(dev) $ just develop

This will compile the library in debug mode and will have poor performance. See Profiling Bytewax for how to compile for proper local benchmarking.

Running a Dataflow#

Once you’ve compiled a changed version of Bytewax, you can run dataflows in exactly the same way as someone who has installed the library normally. See Execution.

Running Tests#

To run the Python or Rust tests use the following recipes.

(dev) $ just test-py
(dev) $ just test-rs

Linting Code#

This will run a series of lints on the code:

  • Ensure all Python code is compatible with at least Python 3.8.

  • Use ruff to lint the Python code.

  • Run mypy to find type errors in the Python code.

  • Run cargo clippy to lint the Rust code.

(dev) $ just lint

Prepare for CI#

CI automatically will run lints, pre-commit hooks, tests, and benchmarks. If you want to run them all locally so there (probably) won’t be any surprises when you push you change, you can use the following recipe.

(dev) $ just ci-pre

Writing Docs#

We have a recipe that will build the docs locally, start up a web server hosting them, and will watch for any changes on Markdown files and re-build the docs.

(dev) $ just doc-autobuild
Running Sphinx v7.2.6
[I 240124 12:03:11 server:335] Serving on

The temporary built HTML files are put in docs/_build/. This directory should not be checked in. Production docs are built using Read the Docs and served from them.

This starts a web server on http://localhost:8000/ with the built docs and will watch the source files and rebuild on any change.


The watching mechanism sometimes gets confused and trapped in an infinite loop, constantly rebuilding the docs on no changes. I think it has something to do with the fact that the Sphinx build process generates Markdown files for the API docs.

If you C-c it and start it again, it will stop.

Adding Dependencies#

For Library#

To add new dependencies that the Bytewax library requires, add them to the dependencies list in pyproject.toml. You should not pin a specific version here, but instead bound by the lowest version that the calling code in Bytewax supports. This is because Bytewax is a library that other people will be including in their applications and so they might have more specific version requirements for their deployment.

After updating, you need to re-compile the deps and commit the changes.

Optional Dependencies#

If there is a feature of the Bytewax library that not everyone will use and requires a new dependency, you can add it as an extra dependency. First add a new extra name to pyproject.toml under the extras list. Then add a list of dependencies under project.optional-dependencies. E.g. for adding Kafka functionality:

extras = [

kafka = [

For Development Environment#

If you want to add a dependency that isn’t required to use the Bytewax library, but now is needed for a fully working development environment, add a line to the appropriate file in requirements/dev.in. The files that end in .in are the “unpinned” description of the top-level requirements.

After updating, you need to re-compile the deps and commit the changes.

Compiling Dependencies#

Whenever you update dependency constraints, we need to lock specific versions of those dependencies so that when we run CI, the environment is reproducible. In a sense, our CI infrastructure is a deployment. This is called compiling the dependencies and we use uv to do that. We have a just recipe that re-compiles all the deps in the correct order.

(dev) $ just venv-compile-all
(dev) $ git add requirements/

Then commit all changes in the requirements/*.txt files.

Upgrading Dependencies#

uv will not modify a requirements/*.txt file just to bump a package version; it will only modify existing locked versions if they no longer pass the constraints of the *.in files, which only specify the coarsest constraints on the package versions. If you want to upgrade all packages to their latest version, you should blow-away all the *.txt files and re-compile them.

(dev) $ rm requirements/*.txt
(dev) $ just venv-compile-all
(dev) $ git add requirements/
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