pyodide/docs/development/new-packages.md

(new-packages)=

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.

Quickstart

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

1. Determining if creating a Pyodide package is necessary

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

If the wheel is not on PyPI, but nevertheless 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 setup.py 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 `setup.py` contains
any compilation commands.

2. Creating the meta.yaml file

If your package is on PyPI, the easiest place to start is with the {ref}mkpkg tool <pyodide-mkpkg>. From the Pyodide root directory, install the tool with pip install ./pyodide-build, then run:

python -m pyodide_build mkpkg <package-name>

This will generate a meta.yaml file under packages/<package-name>/ (see {ref}meta-yaml-spec) 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](https://conda-forge.org/) and open the corresponding `meta.yaml`
file. This can be done either by checking if the URL
`https://github.com/conda-forge/<package-name>-feedstock/blob/master/recipe/meta.yaml`
exists, or by searching the [conda-forge GitHub
org](https://github.com/conda-forge/) for the package name.

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

3. Building the package and investigating issues

Once the meta.yaml file is ready, build the package with the following commands from inside the package directory packages/<package-name>

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>/test_xxx.py. Most packages have one test file named test_<package-name>.py. The tests should look like:

from pyodide_test_runner import run_in_pyodide

@run_in_pyodide(packages=["<package-name>"])
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 pyodide_test_runner 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.

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>-tests.zip
• Repack the wheel with python -m wheel pack

Lastly, a packages.json file is created containing the dependency tree of all packages, so {any}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 {ref}meta-yaml-spec for more details.

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

package:
  name: <name>
  version: <version>

source:
  url: <url>
  sha256: <sha256>

requirements:
  run:
    - <requirement>

build:
  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.

.. toctree::
   :hidden:

   meta-yaml.md