# Creating a Pyodide package Pyodide includes a set of automatic tools to make it easier to add new third-party Python libraries to the build. These tools automate the following steps to build a package: - 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. ## 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](https://conda.io/docs/user-guide/tasks/build-packages/define-metadata.html), 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/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/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: - `$BUILD`: The root of the built package. (`build/lib.XXX/` inside of the source directory). This is what will be installed into Python site-packages. - `$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).