Building from sources

Building is easiest on Linux and relatively straightforward on Mac. For Windows, we currently recommend using the Docker image (described below) to build Pyodide. Another option for building on Windows is to use WSL2 to create a Linux build environment.

Build using make

Make sure the prerequisites for emsdk are installed. Pyodide will build a custom, patched version of emsdk, so there is no need to build it yourself prior.

Additional build prerequisites are:

  • A working native compiler toolchain, enough to build CPython.

  • A native Python 3.8 to run the build scripts.

  • CMake

  • PyYAML

  • FreeType 2 development libraries to compile Matplotlib.

  • Cython to compile SciPy

  • SWIG to compile NLopt

  • gfortran (GNU Fortran 95 compiler)

  • f2c

  • ccache (optional) highly recommended for much faster rebuilds.

On Mac, you will also need:

  • Homebrew for installing dependencies

  • System libraries in the root directory ( sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target / should do it, see

  • coreutils for md5sum and other essential Unix utilities (brew install coreutils)

  • cmake (brew install cmake)

  • Cython to compile SciPy (brew install cython)

  • SWIG to compile NLopt (brew install swig)

  • pkg-config (brew install pkg-config)

  • openssl (brew install openssl)

  • gfortran (brew cask install gfortran)

  • f2c: Install wget (brew install wget), and then run the buildf2c script from the root directory (sudo ./tools/buildf2c)

After installing the build prerequisites, run from the command line:


Using Docker

We provide a Debian-based Docker image on Docker Hub with the dependencies already installed to make it easier to build Pyodide. On top of that we provide a pre-built image which can be used for fast custom and partial builds of Pyodide. Note that building from the non pre-built the Docker image is very slow on Mac, building on the host machine is preferred if at all possible.

  1. Install Docker

  2. From a git checkout of Pyodide, run ./run_docker or ./run_docker --pre-built

  3. Run make to build.

Note: You can control the resources allocated to the build by setting the env vars EMSDK_NUM_CORE, EMCC_CORES and PYODIDE_JOBS (the default for each is 4).

If running make deterministically stops at one point in each subsequent try, increasing the maximum RAM usage available to the docker container might help [This is different from the physical RAM capacity inside the system]. Ideally, at least 3 GB of RAM should be available to the docker container to build Pyodide smoothly. These settings can be changed via Docker Preferences (See here).

You can edit the files in your source checkout on your host machine, and then repeatedly run make inside the Docker environment to test your changes.

Partial builds

To build a subset of available packages in Pyodide, set the environment variable PYODIDE_PACKAGES to a comma separated list of packages. For instance,

PYODIDE_PACKAGES="toolz,attrs" make

Dependencies of the listed packages will be built automatically as well. The package names must match the folder names in packages/ exactly; in particular they are case sensitive.

To build a minimal version of Pyodide, set PYODIDE_PACKAGES="micropip". The packages micropip and distutils are always automatically included (but an empty PYODIDE_PACKAGES is interpreted as unset). As a shorthand for this, one can say make minimal.

Environment variables

Following environment variables additionally impact the build,

  • PYODIDE_JOBS: the -j option passed to the emmake make command when applicable for parallel compilation. Default: 3.

  • PYODIDE_BASE_URL: Base URL where Pyodide packages are deployed. It must end with a trailing /. Default: ./ to load Pyodide packages from the same base URL path as where pyodide.js is located. Example:

  • EXTRA_CFLAGS : Add extra compilation flags.

  • EXTRA_LDFLAGS : Add extra linker flags.

Setting EXTRA_CFLAGS="-D DEBUG_F" provides detailed diagnostic information whenever error branches are taken inside of the Pyodide core code. These error messages are frequently helpful even when the problem is a fatal configuration problem and Pyodide cannot even be initialized. These error branches occur also in correctly working code, but they are relatively uncommon so in practice the amount of noise generated isn’t too large. The shorthand make debug automatically sets this flag.

In certain cases, setting EXTRA_LDFLAGS="-s ASSERTIONS=1 or ASSERTIONS=2 can also be helpful, but this slows down the linking and the runtime speed of Pyodide a lot and generates a large amount of noise in the console.