Creating a Pyodide package

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

  • Download a source tarball (usually from PyPI)
  • Confirm integrity of the package by comparing it to a checksum
  • Apply patches, if any, to the source distribution
  • Add extra files, if any, to the source distribution
  • If the package includes C/C++/Cython extensions:
    • Build the package natively, keeping track of invocations of the native compiler and linker
    • Rebuild the package using emscripten to target WebAssembly
  • If the package is pure Python:
    • Run the setup.py script to get the built package
  • Package the results into an emscripten virtual filesystem package, which comprises:
    • A .data file containing the file contents of the whole package, concatenated together
    • A .js file which contains metadata about the files and installs them into the virtual filesystem.

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

mkpkg

If you wish to create a new package for pyodide, the easiest place to start is with the mkpkg tool. If your package is on PyPI, just run:

bin/pyodide mkpkg $PACKAGE_NAME

This will generate a meta.yaml (see below) that should work out of the box for many pure 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.

The meta.yaml file

Packages are defined by writing a meta.yaml file. The format of these files is based on the meta.yaml files used to build Conda packages, though it is much more limited. The most important limitation is that Pyodide assumes there will only be one version of a given library available, whereas Conda allows the user to specify the versions of each package that they want to install. Despite the limitations, keeping the file format as close as possible to conda’s should make it easier to use existing conda package definitions as a starting point to create Pyodide packages. In general, however, one should not expect Conda packages to “just work” with Pyodide. (In the longer term, Pyodide may use conda as its packaging system, and this should hopefully ease that transition.)

The supported keys in the meta.yaml file are described below.

package

package/name

The name of the package. It must match the name of the package used when expanding the tarball, which is sometimes different from the name of the package in the Python namespace when installed. It must also match the name of the directory in which the meta.yaml file is placed. It can only contain alpha-numeric characters and -, _.

package/version

The version of the package.

source

source/url

The url of the source tarball.

The tarball may be in any of the formats supported by Python’s shutil.unpack_archive: tar, gztar, bztar, xztar, and zip.

source/path

Alternatively to source/url, a relative or absolute path can be specified as package source. This is useful for local testing or building packages which are not available online in the required format.

If a path is specified, any provided checksums are ignored.

source/md5

The MD5 checksum of the tarball. It is recommended to use SHA256 instead of MD5. At most one checksum entry should be provided per package.

source/sha256

The SHA256 checksum of the tarball. It is recommended to use SHA256 instead of MD5. At most one checksum entry should be provided per package.

source/patches

A list of patch files to apply after expanding the tarball. These are applied using patch -p1 from the root of the source tree.

source/extras

Extra files to add to the source tree. This should be a list where each entry is a pair of the form (src, dst). The src path is relative to the directory in which the meta.yaml file resides. The dst path is relative to the root of source tree (the expanded tarball).

build

build/skip_host

Skip building C extensions for the host environment. Default: True.

Setting this to False will result in ~2x slower builds for packages that include C extensions. It should only be needed when a package is a build time dependency for other packages. For instance, numpy is imported during installation of matplotlib, importing numpy also imports included C extensions, therefore it is built both for host and target.

build/cflags

Extra arguments to pass to the compiler when building for WebAssembly.

(This key is not in the Conda spec).

build/ldflags

Extra arguments to pass to the linker when building for WebAssembly.

(This key is not in the Conda spec).

build/post

Shell commands to run after building the library. These are run inside of bash, and there are two special environment variables defined:

  • $SITEPACKAGES: The site-packages directory into which the package has been installed.
  • $PKGDIR: The directory in which the meta.yaml file resides.

(This key is not in the Conda spec).

requirements

requirements/run

A list of required packages.

(Unlike conda, this only supports package names, not versions).