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.

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 alphanumeric 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/extract_dir#

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).

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/cflags#

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

(This key is not in the Conda spec).

build/cxxflags#

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).

build/ldflags#

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

(This key is not in the Conda spec).

build/library#

Should be set to true for library packages. Library packages are packages that are needed for other packages but are not Python packages themselves. For library packages, the script specified in the build/script section is run to compile the library. See the zlib meta.yaml for an example of a library package specification.

build/sharedlibrary#

Should be set to true for shared library packages. Shared library packages are packages that are needed for other packages, but are loaded dynamically when Pyodide is run. For shared library packages, the script specified in the build/script section is run to compile the library. The script should build the shared library and copy it into a subfolder of the source folder called install. Files or folders in this install folder will be packaged to make the Pyodide package. See the CLAPACK meta.yaml for an example of a shared library specification.

build/script#

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 setup.py. This script is run by bash in the directory where the tarball was extracted.

build/post#

Shell commands to run after building the library. These are run with 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).

build/replace-libs#

A list of strings of the form <old_name>=<new_name>, to rename libraries when linking. This in particular might be necessary when using emscripten ports. For instance, png16=png is currently used in matplotlib.

build/unvendor-tests#

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.

requirements#

requirements/run#

A list of required packages.

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

test#

test/imports#

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.