PEP 776: Address some of pfmoore's comments

peps/pep-0776.rst

@@ -635,11 +635,11 @@ 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.
 
-Emscripten Wheel Format
-~~~~~~~~~~~~~~~~~~~~~~~
+Emscripten Wheel Tags
+~~~~~~~~~~~~~~~~~~~~~
 
-Emscripten wheels will use either the format ``emscripten_<version>_wasm32`` or
-``pyodide_<abi>_wasm32``. For example:
+Emscripten wheels will use as the platform tag either
+``emscripten_<version>_wasm32`` or ``pyodide_<abi>_wasm32``. For example:
 
 * ``emscripten_3_1_58_wasm32``
 * ``pyodide_2025_0_wasm32``
@@ -651,6 +651,34 @@ depending on linker and compiler options. It is our intent that the
 ``pyodide_<abi>`` and ``emscripten_<version>`` is intended to be the same as the
 relationship between ``manylinux<version>`` and ``linux``.
 
+For example, wheels with the following tags are compatible with Python 3.13
+Pyodide:
+
+- ``cp13-cp13-emscripten_3_1_58_wasm32``
+- ``cp13-cp13-pyodide_2025_0_wasm32``
+- ``abi3-cp10-pyodide_2025_0_wasm32``
+
+As well as all non platformed tags. To generate the list of compatible tags, one
+can use the following code:
+
+.. code-block:: Python
+
+    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>`__.
+
+Emscripten Wheel ABI
+~~~~~~~~~~~~~~~~~~~~
+
 The specification of the ``pyodide_<abi>`` ABI includes:
 
 * Which version of the Emscripten compiler is used
@@ -675,6 +703,39 @@ with manylinux, there is no need for a Docker container to build the
 versions of Python, Node.js, and Emscripten.
 
 
+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``. We recommend that package indexes continue not
+accepting wheels that match ``emscripten_[0-9]+_[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). Such checks are already in use
+in the wild and seem to be sufficient for the needs of the community.
+
+
+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.
+
+
 PEP 11
 ------
 
@@ -767,7 +828,8 @@ partially-upstreamed features so that Pyodide can replace them with more
 complete versions downstream.
 
 These backwards compatibility concerns impact not just the runtime but also the
-packaging system.
+packaging system. Adding new platform tags should not affect existing packaging
+tools because tools ignore wheels with an unknown package tag.
 
 
 Security Implications
@@ -790,8 +852,11 @@ to use them all at runtime. The documentation will cover this in a similar form
 to the existing Windows embeddable package. In the short term, we will encourage
 developers to use Pyodide if at all possible.
 
-Second, developers of packages with binary components need to know how to build
-and release them for Emscripten (see Packaging).
+Second, maintainers of packages with binary components need to know how to
+build, test, label, and deploy them for Emscripten (see Packaging). The Pyodide
+documentation on `building and testing packages
+<https://pyodide.org/en/stable/development/building-and-testing-packages.html>`__
+is the best reference for this.
 
 
 Reference Implementation