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 support).

Building jaxlib from source

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

  • a C++ compiler (g++ or clang)

  • Numpy

  • Scipy

  • Cython

  • six (required for during the jaxlib build only, not required at install time)

On Ubuntu 18.04 or Debian you can install the necessary prerequisites with:

sudo apt-get install g++ python python3-dev python3-numpy python3-scipy cython3 python3-six

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

You can also install the necessary Python dependencies using pip:

pip install numpy scipy cython six

To build jaxlib with CUDA support, you can run:

python build/ --enable_cuda
pip install -e build  # 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.

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

python build/
pip install -e build  # installs jaxlib (includes XLA)

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 pytest-xdist pytest-benchmark. 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 Travis 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

You must also install pandoc in order to regenerate the notebooks. See Install Pandoc, or using Miniconda which I have used successfully on the Mac: conda install -c conda-forge pandoc. If you do not want to install pandoc then you should regenerate the documentation without the notebooks.

You run at top-level one of the following commands:

sphinx-build -b html docs docs/build/html  # with the notebooks
sphinx-build -b html -D nbsphinx_execute=never docs docs/build/html  # without the notebooks

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

Update notebooks

Open the notebook with (then 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.

Some of the notebooks are built automatically as part of the Travis 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 documentations 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 master 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.

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