Creating a Pyodide package#

It is recommended to look into how other similar packages are built in Pyodide. If you encounter difficulties in building your package after trying the steps listed here, open a new Pyodide issue.

Determining if creating a Pyodide package is necessary#

If you wish to use a package in Pyodide that is not already included in the packages folder, first you need to determine whether it is necessary to package it for Pyodide. Ideally, you should start this process with package dependencies.

Most pure Python packages can be installed directly from PyPI with micropip.install() if they have a pure Python wheel. Check if this is the case by trying micropip.install("package-name").

If there is no wheel on PyPI, but you believe there is nothing preventing it (it is a Python package without C extensions):

  • you can create the wheel yourself by running

    python -m pip install build
    python -m build

    from within the package folder where the are located. See the Python packaging guide for more details. Then upload the wheel file somewhere (not to PyPI) and install it with micropip via its URL.

  • please open an issue in the package repository asking the authors to upload the wheel.

If however the package has C extensions or its code requires patching, then continue to the next steps.


To determine if a package has C extensions, check if its contains any compilation commands.

Building Python wheels (out of tree)#


This feature is still experimental in Pyodide 0.21.0.

It is now possible to build Python wheels for WASM/Emscripten separately from the Pyodide package tree using the following steps,

  1. Install pyodide-build,

    pip install pyodide-build
  2. Build the WASM/Emscripten package wheel by running,

    pyodide build

    in the package folder (where the or pyproject.toml file is located). This command would produce a binary wheel in the dist/ folder, similarly to the PyPa build command.

  3. Make the resulting file accessible as part of your web applications, and install it with micropip.install by URL.

Below is a more complete example for building a Python wheel out of tree with Github Actions CI,

runs-on: ubuntu-latest
  - uses: actions/checkout@v3
  - uses: actions/setup-python@v4
       python-version: 3.10.2
  - uses: mymindstorm/setup-emsdk@v11
       version: 3.1.14
  - run: pip install pyodide-build==0.21.0
  - run: pyodide build


  • the resulting package wheels have a file name of the form *-cp310-cp310-emscripten_3_1_14_wasm32.whl and are compatible only for a given Python and Emscripten versions. In the Pyodide distribution, Python and Emscripten are updated simultaneously.

  • PyPi for now does not support wasm32 wheels so you will not be able to upload them there.

Building a Python package (in tree)#

This section documents how to add a new package to the Pyodide distribution.

1. Creating the meta.yaml file#

To build a Python package, you need to create a meta.yaml file that defines a “recipe” which may include build commands and “patches” (source code edits), amongst other things.

If your package is on PyPI, the easiest place to start is with the mkpkg tool.

First clone and build the Pyodide git repo like this:

git clone
cd pyodide

If you’d like to use a Docker container, you can now run this command:

./run_docker --pre-built

This will mount the current working directory as /src within the container.

Now run make to build the relevant Pyodide tools:


Now install pyodide_build with:

pip install ./pyodide-build

And now you can run mkpkg:

python -m pyodide_build mkpkg <package-name>

This will generate a meta.yaml file under packages/<package-name>/ (see The meta.yaml specification) that should work out of the box for many simple Python packages. This tool will populate the latest version, download link and sha256 hash by querying PyPI. It doesn’t currently handle package dependencies, so you will need to specify those yourself.

You can also use the meta.yaml of other Pyodide packages in the packages/ folder as a starting point.


To reliably determine build and runtime dependencies, including for non Python libraries, it is often useful to verify if the package was already built on conda-forge and open the corresponding meta.yaml file. This can be done either by checking if the URL<package-name>-feedstock/blob/master/recipe/meta.yaml exists, or by searching the conda-forge GitHub org for the package name.

The Pyodide meta.yaml file format was inspired by the one in conda, however it is not strictly compatible.

The package may have special build requirements - e.g. specified in its Github README. If so, you can add extra build commands to the meta.yaml like this:

  script: |

2. Building the package and investigating issues#

Once the meta.yaml file is ready, build the package with the following command

python -m pyodide_build buildall --only 'package-name' packages dist

and see if there are any errors.

If there are errors you might need to

  • patch the package by adding .patch files to packages/<package-name>/patches

  • add the patch files to the source/patches field in the meta.yaml file

then restart the build.

If the build succeeds you can try to load the package by

  1. Serve the dist directory with python -m http.server

  2. Open localhost:<port>/console.html and try to import the package

  3. You can test the package in the repl

Writing tests for your package#

The tests should go in one or more files like packages/<package-name>/ Most packages have one test file named test_<package-name>.py. The tests should look like:

from pytest_pyodide import run_in_pyodide

def test_mytestname(selenium):
  import <package-name>
  assert package.do_something() == 5
  # ...

If you want to run your package’s full pytest test suite and your package vendors tests you can do it like:

from pytest_pyodide import run_in_pyodide

@run_in_pyodide(packages=["<package-name>-tests", "pytest"])
def test_mytestname(selenium):
  import pytest
  pytest.main(["--pyargs", "<package-name>", "-k", "some_filter", ...])

you can put whatever command line arguments you would pass to pytest as separate entries in the list. For more info on run_in_pyodide see pytest-pyodide.

Generating patches#

If the package has a git repository, the easiest way to make a patch is usually:

  1. Clone the git repository of the package. You might want to use the options git clone --depth 1 --branch <version>. Find the appropriate tag given the version of the package you are trying to modify.

  2. Make a new branch with git checkout -b pyodide-version (e.g., pyodide-1.21.4).

  3. Make whatever changes you want. Commit them. Please split your changes up into focused commits. Write detailed commit messages! People will read them in the future, particularly when migrating patches or trying to decide if they are no longer needed. The first line of each commit message will also be used in the patch file name.

  4. Use git format-patch <version> -o <pyodide-root>/packages/<package-name>/patches/ to generate a patch file for your changes and store it directly into the patches folder.

Migrating Patches#

When you want to upgrade the version of a package, you will need to migrate the patches. To do this:

  1. Clone the git repository of the package. You might want to use the options git clone --depth 1 --branch <version-tag>.

  2. Make a new branch with git checkout -b pyodide-old-version (e.g., pyodide-1.21.4).

  3. Apply the current patches with git am <pyodide-root>/packages/<package-name>/patches/*.

  4. Make a new branch git checkout -b pyodide-new-version (e.g., pyodide-1.22.0)

  5. Rebase the patches with git rebase old-version --onto new-version (e.g., git rebase pyodide-1.21.4 --onto pyodide-1.22.0). Resolve any rebase conflicts. If a patch has been upstreamed, you can drop it with git rebase --skip.

  6. Remove old patches with rm <pyodide-root>/packages/<package-name>/patches/*.

  7. Use git format-patch <version-tag> -o <pyodide-root>/packages/<package-name>/patches/ to generate new patch files.

Upstream your patches!#

Please create PRs or issues to discuss with the package maintainers to try to find ways to include your patches into the package. Many package maintainers are very receptive to including Pyodide-related patches and they reduce future maintenance work for us.

The package build pipeline#

Pyodide includes a toolchain to add new third-party Python libraries to the build. We automate the following steps:

  • If source is a url (not in-tree):

    • Download a source archive or a pure python wheel (usually from PyPI)

    • Confirm integrity of the package by comparing it to a checksum

    • If building from source (not from a wheel):

      • Apply patches, if any, to the source distribution

      • Add extra files, if any, to the source distribution

  • If the source is not a wheel (building from a source archive or an in-tree source):

    • Run build/script if present

    • Modify the PATH to point to wrappers for gfortran, gcc, g++, ar, and ld that preempt compiler calls, rewrite the arguments, and pass them to the appropriate emscripten compiler tools.

    • Using pypa/build:

      • Create an isolated build environment. Install symbolic links from this isolated environment to “host” copies of certain unisolated packages.

      • Install the build dependencies requested in the package build-requires. (We ignore all version constraints on the unisolated packages, but version constraints on other packages are respected.

      • Run the PEP 517 build backend associated to the project to generate a wheel.

  • Unpack the wheel with python -m wheel unpack.

  • Run the build/post script in the unpacked wheel directory if it’s present.

  • Unvendor unit tests included in the installation folder to a separate zip file <package name>

  • Repack the wheel with python -m wheel pack

Lastly, a repodata.json file is created containing the dependency tree of all packages, so pyodide.loadPackage can load a package’s dependencies automatically.

Partial Rebuilds#

By default, each time you run buildpkg, pyodide-build will delete the entire source directory and replace it with a fresh copy from the download url. This is to ensure build repeatability. For debugging purposes, this is likely to be undesirable. If you want to try out a modified source tree, you can pass the flag --continue and buildpkg will try to build from the existing source tree. This can cause various issues, but if it works it is much more convenient.

Using the --continue flag, you can modify the sources in tree to fix the build, then when it works, copy the modified sources into your checked out copy of the package source repository and use git format-patch to generate the patch.

C library dependencies#

Some Python packages depend on certain C libraries, e.g. lxml depends on libxml.

To package a C library, create a directory in packages/ for the C library. In the directory, you should write meta.yaml that specifies metadata about the library. See The meta.yaml specification for more details.

The minimal example of meta.yaml for a C library is:

  name: <name>
  version: <version>

  url: <url>
  sha256: <sha256>

    - <requirement>

  library: true
  script: |
    emconfigure ./configure
    emmake make -j ${PYODIDE_JOBS:-3}

You can use the meta.yaml of other C libraries such as libxml as a starting point.

After packaging a C library, it can be added as a dependency of a Python package like a normal dependency. See lxml and libxml for an example (and also scipy and CLAPACK).

Remark: Certain C libraries come as emscripten ports, and do not have to be built manually. They can be used by adding e.g. -s USE_ZLIB in the cflags of the Python package. See e.g. matplotlib for an example. The full list of libraries with Emscripten ports is here.

Structure of a Pyodide package#

Pyodide is obtained by compiling CPython into WebAssembly. As such, it loads packages the same way as CPython — it looks for relevant files .py and .so files in the directories in sys.path. When installing a package, our job is to install our .py and .so files in the right location in emscripten’s virtual filesystem.

Wheels are just zip archives, and to install them we unzip them into the site-packages directory. If there are any .so files, we also need to load them at install time: WebAssembly must be loaded asynchronously, but Python imports are synchronous so it is impossible to load .so files lazily.

Rust/PyO3 Packages#

We currently build cryptography which is a Rust extension built with PyO3 and setuptools-rust. It should be reasonably easy to build other Rust extensions. Currently it is necessary to run source $CARGO_HOME/env in the build script as shown here, but other than that there may be no other issues if you are lucky.

As mentioned here, by default certain wasm-related RUSTFLAGS are set during build.script and can be removed with export RUSTFLAGS="".