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Update Documentation for Emscripten Builds #1621

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150a271
update documentation for emscripten builds
adqm Aug 2, 2025
7a7c32a
remove emsdk version suggestion for cpython 3.13
adqm Aug 3, 2025
144a72f
add make-mpdec step to the broken-out build commands
adqm Aug 3, 2025
703a7a5
suggest python3 for Tools/wasm/emscripten commands
adqm Aug 3, 2025
9fee316
bump EMSDK version for v3.14
adqm Aug 13, 2025
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41 changes: 29 additions & 12 deletions getting-started/setup-building.rst
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Expand Up @@ -493,13 +493,26 @@ The simplest way to install the Emscripten compiler is:

# Install Emscripten
git clone https://github.com/emscripten-core/emsdk
./emsdk/emsdk install 4.0.5
./emsdk/emsdk activate 4.0.5
./emsdk/emsdk install 4.0.12
./emsdk/emsdk activate 4.0.12
source ./emsdk/emsdk_env.sh

Updating the Emscripten compiler version often causes breakages. For the best
compatibility, use the Emscripten version suggested in the cpython repository in
``Tools/wasm/README.md``.
Updating the Emscripten compiler version can cause breakages. For the best
compatibility, use the appropriate Emscripten version based on the version of
CPython you're building:

* For building CPython 3.14, use ``emsdk`` version ``4.0.12``.
* For building the main branch of the CPython repository, you may wish to use
``latest`` instead of a specific version.

It is possible (but not necessary) to enable ``ccache`` for Emscripten builds
by setting the ``EM_COMPILER_WRAPPER`` environment, but this step will only
take effect if it is done **after** ``emsdk_env.sh`` is sourced (otherwise, the
sourced script removes the environment variable):
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@hoodmane This was the historical guidance for ccache... is it still required now that the --config-cache option is part of the emscripten build script?

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@hoodmane hoodmane Aug 14, 2025 •
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Yes, compilation cache and configure cache are different things.


.. code-block:: sh

export EM_COMPILER_WRAPPER=ccache

Building for Emscripten requires doing a cross-build where you have a *build*
Python to help produce an Emscripten build of CPython. This means you build
Expand All @@ -508,8 +521,8 @@ another that's the build you ultimately care about (that is, the build Python is
not meant for use by you directly, only the build system).

The easiest way to get a debug build of CPython for Emscripten is to use the
``Tools/wasm/emscripten build`` command (which should be run with a recent
version of Python you have installed on your machine):
``Tools/wasm/emscripten build`` command, which should be run with a recent
version of Python (3.13 or newer) already installed on your machine:
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I've been running it with 3.12 but it's harmless to say 3.13 here.


.. code-block:: shell

Expand All @@ -524,11 +537,12 @@ is a convenience wrapper around the following commands:

.. code-block:: shell

python Tools/wasm/emscripten configure-build-python --quiet -- --config-cache --with-pydebug
python Tools/wasm/emscripten make-build-python --quiet
python Tools/wasm/emscripten make-libffi --quiet
python Tools/wasm/emscripten configure-host --quiet -- --config-cache
python Tools/wasm/emscripten make-host --quiet
python3 Tools/wasm/emscripten configure-build-python --quiet -- --config-cache --with-pydebug
python3 Tools/wasm/emscripten make-build-python --quiet
python3 Tools/wasm/emscripten make-libffi --quiet
python3 Tools/wasm/emscripten make-mpdec --quiet
python3 Tools/wasm/emscripten configure-host --quiet -- --config-cache
python3 Tools/wasm/emscripten make-host --quiet

.. note::

Expand All @@ -554,6 +568,9 @@ used in ``python.sh``:

make -C cross-build/wasm32-emscripten/build/python/ test

Additional instructions for running the resulting builds (through Node.js and/or
through web browsers) are available in the CPython repository at
:cpy-file:`Tools/wasm/README.md`.

.. _Emscripten: https://emscripten.org/
.. _WebAssembly: https://webassembly.org
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