The meta.yaml specification#

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, it is recommended 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, see #795

This is unstable

The Pyodide build system is under fairly active development (as of 2022/03/13). The next couple of releases are likely to include breaking changes.

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



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 alphanumeric characters, -, and _.


The version of the package.


The list of top-level import name for the package. This key is used in pyodide.loadPackagesFromImports(). For example, the top-level import name for the scikit-learn is sklearn. Some packages may have multiple top-level import names. For instance, setuptools exposes setuptools and pkg_resources as a top-level import names.


The list of tags of the package. This is meta information used to group packages by functionality. Normally this is not needed. The following tags are currently used in Pyodide:

  • always: This package is always built.

  • core: This package is used in the Pyodide core test suite.

  • min-scipy-stack: This package is part of the minimal scipy stack.



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.


The top level directory name of the contents of the source tarball (i.e. once you extract the tarball, all the contents are in the directory named source/extract_dir). This defaults to the tarball name (sans extension).


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.


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.


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


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).



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

(This key is not in the Conda spec).


Extra arguments to pass to the compiler when building C++ files for WebAssembly. Note that both cflags and cxxflags will be used when compiling C++ files. A common example would be to use -std=c++11 for code that makes use of C++11 features.

(This key is not in the Conda spec).


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

(This key is not in the Conda spec).


Which symbols should be exported from the shared object files. Possible values are:

  • pyinit: The default. Only export Python module initialization symbols of the form PyInit_some_module.

  • requested: Export the functions that are marked as exported in the object files. Switch to this if pyinit doesn’t work. Useful for packages that use ctypes or dlsym to access symbols.

  • whole_archive: Uses -Wl,--whole-archive to force inclusion of all symbols. Use this when neither pyinit nor explicit work.


Extra flags to pass to the build backend (e.g., setuptools, flit, etc).


Type of the package. Possible values are:

  • package (default): A normal Python package, built to a wheel file.

  • static_library: A static library.

  • shared_library: A shared library.

  • cpython_module: A CPython stdlib extension module. This is used for unvendoring CPython modules, and should not be used for other purposes.

If you are building ordinary Python package, you don’t need to set this key. But if you are building a static or shared library, you need to set this to static_library or shared_library respectively.

Static and shared libraries are not Python packages themselves, but are needed for other python packages. For libraries, the script specified in the build/script section is run to compile the library.

The difference between static_library and shared_library is that static_library is statically linked into the other packages, so it is required only in the build time, while shared_library is dynamically linked, so it is required in the runtime. When building a shared library, you should copy the built libraries into the $DISTDIR. Files or folders in this folder will be packaged to make the Pyodide package.

See the zlib meta.yaml for an example of a static library specification, and the OpenBLAS meta.yaml for an example of a shared library specification.


The script section is required for a library package (build/library set to true). For a Python package this section is optional. If it is specified for a Python package, the script section will be run before the build system runs This script is run by bash in the directory where the tarball was extracted.

There are special environment variables defined:

  • $PKGDIR: The directory in which the meta.yaml file resides.

  • $PKG_VESRION: The version of the package

  • $PKG_BUILD_DIR: The directory where the tarball was extracted.

  • $DISTDIR: The directory where the built wheel or library should be placed. If you are building a shared library, you should copy the built libraries into this directory.

(These keys are not in the Conda spec).


This script will run after build/script. The difference is that it runs with the target environment variables and sysconfigdata and with the pywasmcross compiler symlinks. Any changes to the environment will persist to the main build step but will not be seen in the build/post step (or anything else done outside of the cross build environment). The working directory for this script is the source directory.


Shell commands to run after building the library. This script is run by bash in the directory where meta.yaml file resides. The ${PKG_BUILD_DIR}/dist will contain the built wheel unpacked with python -m wheel unpack so it’s possible to manually add, delete, change, move files etc. See the [setuptools meta.yaml]( blob/main/packages/setuptools/meta.yaml) for an example of the usage of this key.


Whether to unvendor tests found in the installation folder to a separate package <package-name>-tests. If this option is true and no tests are found, the test package will not be created. Default: true.


If set to true, shared libraries that are required by the package will be vendored into the package after the build. This is similar to what auditwheel repair does, but it is done in a way that is compatible with Pyodide and Emscripten dynamic linking. Default: false.



A list of required packages at runtime.

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


A list of Pyodide packages that are required when building a package. It represents packages that need to be specific to the target platform.

For instance, when building libxml, zlib needs to be built for WASM first, and so it’s a host dependency. This is unrelated to the fact that the build system might already have zlib present.


A list of executables that are required when building a package.

Note that unlike conda, specifying executables in this key doesn’t actually install any of them. This key exists to halt build earlier if required executables are not available.



List of imports to test after the package is built.

Supported Environment Variables#

The following environment variables can be used in the scripts in the meta.yaml files:

  • PYODIDE_ROOT: The path to the base Pyodide directory

  • PYMAJOR: Current major Python version

  • PYMINOR: Current minor Python version

  • PYMICRO: Current micro Python version

  • SIDE_MODULE_CFLAGS: The standard CFLAGS for a side module. Use when compiling libraries or shared libraries.

  • SIDE_MODULE_LDFLAGS: The standard LDFLAGS for a side module. Use when linking a shared library.

  • NUMPY_LIB: Use -L$NUMPY_LIB as a ldflag when linking -lnpymath or -lnpyrandom.