Building and testing Python packages out of tree#

This is some information about how to build and test Python packages against Pyodide out of tree (for instance in your package’s CI or for use with private packages).

Building binary packages for Pyodide#

If your package is a pure Python package (i.e., if the wheel ends in py3-none-any.whl) then follow the official PyPA documentation on building wheels Otherwise, the procedure is simple. In your package directory run the following command line commands:

pip install pyodide-build
pyodide build

Pyodide currently only supports Linux for out of tree builds, though there is a good change it will work in MacOS too. If you are using Windows, try Windows Subsystem for Linux.

pyodide build invokes a slightly modified version of the pypa/build build frontend so the behavior should be similar to what happens if you do:

pip install build
python -m build

If you run into problems, make sure that building a native wheel with pypa/build works.

pyodide build respects the environment variables CFLAGS, CXXFLAGS, and LDFLAGS, so if you need to customize compiler arguments you can set these. Any additional arguments passed to pyodide build will be passed along to the build backend.

If you run into problems, please open an issue about it.

Testing packages against Pyodide#

Pyodide provides an experimental command line runner for testing packages against Pyodide. Using it requires nodejs version 14 or newer.

The way it works is simple: you can create a virtual environment with:

pyodide venv .venv-pyodide

Activate it just like a normal virtual environment:

source .venv-pyodide/bin/python

As a warning, things are pretty weird inside of the Pyodide virtual environment because python points to the Pyodide Python runtime. Any program that uses Python and is sensitive to the current virtual environment will probably break.

You can install whatever dependencies you need with pip. For a pure Python package, the following will work:

pip install -e .

For a binary package, you will need to build a wheel with `pyodide build` and
then point `pip` directly to the built wheel. For now, editable installs won't
work with binary packages.

# Build the binary package
pyodide build
# Install it
pip install dist/the_wheel-cp310-cp310-emscripten_3_1_20_wasm32.whl[tests]

To test, you can generally run the same script as you would usually do. For many packages this will be:

python -m pytest

but for instance numpy uses a file called; the following works:


and you can pass options to it just like normal. Currently subprocess doesn’t work, so if you have a test runner that uses subprocess then it cannot be used.