Maintaining Pyodide support

Last updated: August 2026

This page describes how to update cibuildwheel's Pyodide platform code when either:

  • a new Pyodide alpha release arrives with support for a new PyEmscripten Platform (which is tied to updates in Emscripten and CPython versions, compiler/linker flags, and so on), or
  • when that alpha release graduates to a stable one.

Background

Pyodide has three types of releases that matter to cibuildwheel:

  • Stable – the most recent full Pyodide release. This is enabled by default with no special CIBW_ENABLE flag needed.
  • Prerelease – an alpha/beta/rc Pyodide release that uses the next CPython version. Users must opt in with CIBW_ENABLE: cpython-prerelease to build against this version. This may or may not be available at any given time, depending on the Pyodide release cycle.
  • End-of-life (EoL) – older Pyodide stable releases that are no longer the current stable. These are kept available behind CIBW_ENABLE: pyodide-eol so that users who still need to build for older Pyodide versions can do so.

The guards in cibuildwheel/selector.py enforce this distinction. The constraints files under cibuildwheel/resources/ pin the exact tool versions that go with each build.

When a new Pyodide prerelease becomes available

For example, consider a scenario when Pyodide ships a new 315.0.0a1 with cp315 support.

1. Add the new Python configuration

In cibuildwheel/resources/build-platforms.toml, add an entry under [pyodide]:

{ identifier = "cp315-pyodide_wasm32", version = "3.15", default_pyodide_version = "315.0.0a1", node_version = "v24", sha256 = "SHA256" },

version is the CPython version string, default_pyodide_version is the Pyodide release to use when the user does not pin one explicitly (use the latest available alpha/beta for a prerelease entry), and node_version is the minimum Node.js major required by that Pyodide release — check the pyodide-build FAQ for a rudimentary idea of what the correct value is. sha256 is the checksum of the Pyodide xbuildenv tarball.

2. Generate and pin a constraints file

Run the update_constraints nox session, which reads build-platforms.toml and regenerates all Pyodide constraints files automatically:

nox -s update_constraints

3. Update tests

Update the unit tests so the new identifier is accepted by the selector with CPythonPrerelease enabled and rejected without it. Pyodide-specific integration tests may also need their hardcoded expected-wheel lists extended.

When an old Pyodide version is to be moved to end-of-life

When a Pyodide version is superseded by a new stable release, move it behind the pyodide-eol enable flag. We want to allow users who still build for older Pyodide ABIs time to upgrade.

1. Add the pyodide-eol guard in the selector

In cibuildwheel/selector.py, add (or update) the PyodideEoL guard to include the old identifier:

if EnableGroup.PyodideEoL not in self.enable and fnmatch(build_id, "cp312-pyodide_*"):
    return False

2. Update tests

Update the unit tests so the EoL identifier requires PyodideEoL to be included in the enable set. The default (no CIBW_ENABLE) should exclude it.

When an old Pyodide version is to be fully retired

Retirement is not expected to happen on a routine basis. It is only warranted when the Pyodide ecosystem itself has evolved to the point where an older ABI version is considered obsolete – for example, if the surrounding toolchain, packaging standards, or runtime infrastructure have moved on so substantially that building for the older version no longer makes practical sense. Any retirement is to be discussed and agreed upon by Pyodide maintainers before proceeding.

1. Remove the Python configuration

Delete the entry from build-platforms.toml and remove the PyodideEoL guard for that identifier in selector.py.

2. Delete the constraints file

Remove cibuildwheel/resources/constraints-pyodideXYZ.txt.

3. Update tests

Remove references to the old identifier from the unit tests, integration tests, and drop any expected-wheel entries for it from the test helper.