Building from source

First, obtain the JAX source code:

git clone
cd jax

Building JAX involves two steps:

  1. Building or installing jaxlib, the C++ support library for jax.

  2. Installing the jax Python package.

Building or installing jaxlib

Installing jaxlib with pip

If you’re only modifying Python portions of JAX, we recommend installing jaxlib from a prebuilt wheel using pip:

pip install jaxlib

See the JAX readme for full guidance on pip installation (e.g., for GPU and TPU support).

Building jaxlib from source

To build jaxlib from source, you must also install some prerequisites:

  • a C++ compiler (g++, clang, or MSVC)

    On Ubuntu or Debian you can install the necessary prerequisites with:

    sudo apt install g++ python python3-dev

    If you are building on a Mac, make sure XCode and the XCode command line tools are installed.

    See below for Windows build instructions.

  • Python packages: numpy, six, wheel.

    The six package is required for during the jaxlib build only, and is not required at install time.

You can install the necessary Python dependencies using pip:

pip install numpy six wheel

To build jaxlib with CUDA support, you can run:

python build/ --enable_cuda
pip install dist/*.whl  # installs jaxlib (includes XLA)

See python build/ --help for configuration options, including ways to specify the paths to CUDA and CUDNN, which you must have installed. Here python should be the name of your Python 3 interpreter; on some systems, you may need to use python3 instead. By default, the wheel is written to the dist/ subdirectory of the current directory.

To build jaxlib without CUDA GPU support (CPU only), drop the --enable_cuda:

python build/
pip install dist/*.whl  # installs jaxlib (includes XLA)

Additional Notes for Building jaxlib from source on Windows

On Windows, follow Install Visual Studio to set up a C++ toolchain. Visual Studio 2019 version 16.5 or newer is required. If you need to build with CUDA enabled, follow the CUDA Installation Guide to set up a CUDA environment.

You can either install Python using its Windows installer, or if you prefer, you can use Anaconda or Miniconda to set up a Python environment.

Some targets of Bazel use bash utilities to do scripting, so MSYS2 is needed. See Installing Bazel on Windows for more details. Install the following packages:

pacman -S patch coreutils

Once coreutils is installed, the realpath command should be present in your shell’s path.

Once everything is installed. Open PowerShell, and make sure MSYS2 is in the path of the current session. Ensure bazel, patch and realpath are accessible. Activate the conda environment. The following command builds with CUDA enabled, adjust it to whatever suitable for you:

python .\build\ `
  --enable_cuda `
  --cuda_path='C:/Program Files/NVIDIA GPU Computing Toolkit/CUDA/v10.1' `
  --cudnn_path='C:/Program Files/NVIDIA GPU Computing Toolkit/CUDA/v10.1' `
  --cuda_version='10.1' `

To build with debug information, add the flag --bazel_options='--copt=/Z7'.

Installing jax

Once jaxlib has been installed, you can install jax by running:

pip install -e .  # installs jax

To upgrade to the latest version from GitHub, just run git pull from the JAX repository root, and rebuild by running or upgrading jaxlib if necessary. You shouldn’t have to reinstall jax because pip install -e sets up symbolic links from site-packages into the repository.

Running the tests

To run all the JAX tests, we recommend using pytest-xdist, which can run tests in parallel. First, install pytest-xdist and pytest-benchmark by running pip install -r build/test-requirements.txt. Then, from the repository root directory run:

pytest -n auto tests

JAX generates test cases combinatorially, and you can control the number of cases that are generated and checked for each test (default is 10). The automated tests currently use 25:

JAX_NUM_GENERATED_CASES=25 pytest -n auto tests

The automated tests also run the tests with default 64-bit floats and ints:

JAX_ENABLE_X64=1 JAX_NUM_GENERATED_CASES=25 pytest -n auto tests

You can run a more specific set of tests using pytest’s built-in selection mechanisms, or alternatively you can run a specific test file directly to see more detailed information about the cases being run:

python tests/ --num_generated_cases=5

You can skip a few tests known as slow, by passing environment variable JAX_SKIP_SLOW_TESTS=1.

To specify a particular set of tests to run from a test file, you can pass a string or regular expression via the --test_targets flag. For example, you can run all the tests of jax.numpy.pad using:

python tests/ --test_targets="testPad"

The Colab notebooks are tested for errors as part of the documentation build.

Note that to run the full pmap tests on a (multi-core) CPU-only machine, you can run:

pytest tests/

I.e. don’t use the -n auto option, since that effectively runs each test on a single-core worker.

Type checking

We use mypy to check the type hints. To check types locally the same way as the CI checks them:

pip install mypy
mypy --config=mypy.ini --show-error-codes jax

Update documentation

To rebuild the documentation, install several packages:

pip install -r docs/requirements.txt

And then run:

sphinx-build -b html docs docs/build/html

This can take a long time because it executes many of the notebooks in the documentation source; if you’d prefer to build the docs without executing the notebooks, you can run:

sphinx-build -b html -D jupyter_execute_notebooks=off docs docs/build/html

You can then see the generated documentation in docs/build/html/index.html.

Update notebooks

We use jupytext to maintain two synced copies of the notebooks in docs/notebooks: one in ipynb format, and one in md format. The advantage of the former is that it can be opened and executed directly in Colab; the advantage of the latter is that it makes it much easier to track diffs within version control.

Editing ipynb

For making large changes that substantially modify code and outputs, it is easiest to edit the notebooks in Jupyter or in Colab. To edit notebooks in the Colab interface, open and Upload from your local repo. Update it as needed, Run all cells then Download ipynb. You may want to test that it executes properly, using sphinx-build as explained above.

Editing md

For making smaller changes to the text content of the notebooks, it is easiest to edit the .md versions using a text editor.

Syncing notebooks

After editing either the ipynb or md versions of the notebooks, you can sync the two versions using jupytext by running:

$ jupytext --sync docs/notebooks/*

Alternatively, you can run this command via the pre-commit framework by executing the following in the main JAX directory:

$ pre-commit run --all

See the pre-commit framework documentation for information on how to set your local git environment to execute this automatically.

Creating new notebooks

If you are adding a new notebook to the documentation and would like to use the jupytext --sync command discussed here, you can set up your notebook for jupytext by using the following command:

$ jupytext --set-formats ipynb,md:myst path/to/the/notebook.ipynb

This works by adding a "jupytext" metadata field to the notebook file which specifies the desired formats, and which the jupytext --sync command recognizes when invoked.

Notebooks within the sphinx build

Some of the notebooks are built automatically as part of the pre-submit checks and as part of the Read the docs build. The build will fail if cells raise errors. If the errors are intentional, you can either catch them, or tag the cell with raises-exceptions metadata (example PR). You have to add this metadata by hand in the .ipynb file. It will be preserved when somebody else re-saves the notebook.

We exclude some notebooks from the build, e.g., because they contain long computations. See exclude_patterns in

Documentation building on

JAX’s auto-generated documentation is at

The documentation building is controlled for the entire project by the readthedocs JAX settings. The current settings trigger a documentation build as soon as code is pushed to the GitHub main branch. For each code version, the building process is driven by the .readthedocs.yml and the docs/ configuration files.

For each automated documentation build you can see the documentation build logs.

If you want to test the documentation generation on Readthedocs, you can push code to the test-docs branch. That branch is also built automatically, and you can see the generated documentation here. If the documentation build fails you may want to wipe the build environment for test-docs.

For a local test, I was able to do it in a fresh directory by replaying the commands I saw in the Readthedocs logs:

mkvirtualenv jax-docs  # A new virtualenv
mkdir jax-docs  # A new directory
cd jax-docs
git clone --no-single-branch --depth 50
cd jax
git checkout --force origin/test-docs
git clean -d -f -f
workon jax-docs

python -m pip install --upgrade --no-cache-dir pip
python -m pip install --upgrade --no-cache-dir -I Pygments==2.3.1 setuptools==41.0.1 docutils==0.14 mock==1.0.1 pillow==5.4.1 alabaster>=0.7,<0.8,!=0.7.5 commonmark==0.8.1 recommonmark==0.5.0 'sphinx<2' 'sphinx-rtd-theme<0.5' 'readthedocs-sphinx-ext<1.1'
python -m pip install --exists-action=w --no-cache-dir -r docs/requirements.txt
cd docs
python `which sphinx-build` -T -E -b html -d _build/doctrees-readthedocs -D language=en . _build/html