PEP tbd: Emscripten packaging

Author: hoodmane

peps/pep-tbd.rst

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+PEP: tbd
+Title: Emscripten Packaging
+Author: Hood Chatham <roberthoodchatham at gmail.com>
+Sponsor: Łukasz Langa <lukasz at python.org>
+Discussions-To: 
+Status: Draft
+Type: Standards Track
+Topic: Packaging
+Created: 28-Mar-2025
+Python-Version: 3.14
+Post-History: 
+
+Abstract
+========
+
+This PEP proposes a new platform tag series `pyodide` for binary Python package
+distributions for the Pyodide Python runtime.
+
+`Emscripten <https://emscripten.org/>`__ is a complete open source compiler
+toolchain. It compiles C/C++ code into WebAssembly/JavaScript executables, for
+use in JavaScript runtimes, including browsers and Node.js. The Rust language
+also maintains an Emscripten target.
+
+The goals of this PEP are:
+
+1. To describe Pyodide's packaging tooling
+2. To describe Pyodide's wheel abi to a similar level of detail as the manylinux
+   PEPs
+3. For Pyodide wheels to be allowed for upload to PyPI
+
+
+
+Motivation
+==========
+
+Pyodide is a CPython distribution for use in the browser. A web browser is a
+universal computing platform, available on Windows, macOS, Linux, and every
+smartphone. Hundreds of thousands of students have learned Python through
+Pyodide via projects like `Capytale
+<https://web.archive.org/web/20241211090946/https://cfp.jupytercon.com/2023/talk/TJ9YEV/>`__
+and `PyodideU <https://stanford.edu/~cpiech/bio/papers/pyodideU.pdf>`__. Pyodide
+is also increasingly being used by Python packages to provide interactive
+documentation.
+
+Pyodide currently maintains ports of 255 different packages at the time of this
+writing, including major scientific Python packages like NumPy, SciPy, pandas,
+Polars, scikit-learn, OpenCV, PyArrow, and Pillow as well as general purpose
+packages like aiohttp, Requests, Pydantic, cryptography, and orjson.
+
+About 60 packages are also testing against Pyodide in their CI, including NumPy,
+pandas, awkward-cpp, scikit-image, statsmodels, PyArrow, Hypothesis, and PyO3.
+
+Python package projects cannot deploy binary distributions for Pyodide on PyPI.
+Instead they must use other options like ``anaconda.org`` or ``jsdelivr.com``.
+This creates friction both for package maintainers and for users.
+
+
+Rationale
+=========
+
+Emscripten uses a variant of musl libc. The Emscripten compiler makes no ABI
+stability guarantees between versions. Many Emscripten updates are ABI
+compatible by chance, and the Rust Emscripten target behaves as if the ABI were
+stable with only `occasional negative consequences
+<https://github.com/rust-lang/rust/issues/131467>`__.
+
+There are several linker flags that adjust the Emscripten ABI, so Python
+packages built to run with Emscripten must make sure to match the ABI-sensitive
+linker flags used to compile the interpreter to avoid load-time or run-time
+errors. The Emscripten compiler continuously fixes bugs and adds support for new
+web platform features. Thus, there is significant benefit to being able to
+update the ABI.
+
+In order to balance the ABI stability needs of package maintainers with the ABI
+flexibility to allow the platform to move forward, Pyodide plans to adopt a new
+ABI for each feature release of Python.
+
+The Pyodide team also coordinates the ABI flags that Pyodide uses with the
+Emscripten ABI that Rust supports in order to ensure that we have support for
+the many popular Rust packages. Historically, most of the work for this has
+been related to unwinding ABIs. See for instance `this Rust Major Change
+Proposal <https://github.com/rust-lang/compiler-team/issues/801>`__.
+
+The ``pyodide`` platform tags only apply to Python interpreters compiled and
+linked with the same version of Emscripten as Pyodide, with the same
+ABI-sensitive flags.
+
+
+Specification
+=============
+
+The platform tags will take the form::
+
+    pyodide_${YEAR}_${PATCH}_wasm32
+
+Each one of these will be used with a specified Python version. For example, the
+platform tag `pyodide_2025_0` will be used with Python 3.13. 
+
+Emscripten Wheel ABI
+--------------------
+
+The specification of the ``pyodide_<abi>`` platform includes:
+
+* Which version of the Emscripten compiler is used
+* What libraries are statically linked with the interpreter
+* What stack unwinding ABI is to be used
+* Which runtime platform features are required to be present
+
+and a handful of other similar details that affect the ABI.
+
+The ABI is selected by choosing the appropriate version of the Emscripten
+compiler and passing appropriate compiler and linker flags. It is possible for
+other people to build their own Python interpreter that is compatible with the
+Pyodide ABI, it is not necessary to use the Pyodide distribution itself.
+
+The ``pyodide build`` tool knows how to create wheels that match our ABI. Unlike
+with manylinux wheels, there is no need for a Docker container to build the
+``pyodide_<abi>`` wheels. All that is needed is a Linux machine and appropriate
+versions of Python, Node.js, and Emscripten.
+
+It is possible to validate a wheel by installing and importing it into the
+Pyodide runtime. Because Pyodide can run in an environment with strong
+sandboxing guarantees, doing this produces no security risks.
+
+Determining the ABI version
+---------------------------
+
+The Pyodide ABI version is stored in the `PYODIDE_ABI_VERSION` config variable
+and can be determined via::
+
+    pyodide_abi_version = sysconfig.get_config_var("PYODIDE_ABI_VERSION")
+
+
+To generate the list of compatible tags, one can use the following code::
+
+    from packaging.tags import cpython_tags, _generic_platforms
+
+    def _emscripten_platforms() -> Iterator[str]:
+        pyodide_abi_version = sysconfig.get_config_var("PYODIDE_ABI_VERSION")
+        if pyodide_abi_version:
+            yield f"pyodide_{pyodide_abi_version}_wasm32"
+        yield from _generic_platforms()
+
+    emscripten_tags = cpython_tags(platforms=_emscripten_platforms())
+
+This code will be added to `pypa/packaging
+<https://github.com/pypa/packaging/pull/804>`__.
+
+
+Package Installers
+------------------
+
+Installers should use the ``_emscripten_platforms()`` function shown above to
+determine which platforms are compatible with an Emscripten build of CPython. In
+particular, the Pyodide ABI version is exposed via
+``sysconfig.get_config_var("PYODIDE_ABI_VERSION")``.
+
+Package indexes
+---------------
+
+We recommend that package indexes accept any wheel whose platform tag matches
+``pyodide_[0-9]+_[0-9]+_wasm32``.
+
+
+Dependency Specifier Markers
+----------------------------
+
+To check for the Emscripten platform in a dependency specifier, one can use
+``sys_platform == 'emscripten'`` (or its negation).
+
+
+Trove Classifier
+----------------
+
+Packages that build and test Emscripten wheels can declare this by adding the
+``Environment :: WebAssembly :: Emscripten``. PyPI already accepts uploads of
+packages with this classifier.
+
+
+Backwards Compatibility
+=======================
+
+There are no backwards compatibility concerns in this PEP.
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.