fcx_image

The guest image of FreeCAD's expression sandbox, compiled for Pyodide.

FreeCAD evaluates expressions and expression-language programs that are carried inside a document in a WebAssembly sandbox rather than in the host interpreter, so a file from an untrusted source cannot reach the host's Python, its files or its network. fcx_image is the guest half of that arrangement: a wasm32-emscripten extension module that runs inside Pyodide and implements the curated surface the host exposes to sandboxed code.

This is not a package you install

It is not importable on a normal CPython host, and a plain pip install fcx-image will not find a matching wheel. The file is loaded by FreeCAD's own Pyodide runtime through loadPackage(), from <FreeCAD user data>/Pyodide/wheels/. FreeCAD downloads and verifies the Pyodide runtime itself; this wheel is the piece built from FreeCAD's own sources.

To fetch it deliberately, which is what a packager does:

pip download --no-deps --only-binary=:all: \
    --platform pyemscripten_2026_0_wasm32 \
    --python-version 3.14 --implementation cp --abi cp314 \
    fcx-image

One step is needed after downloading: rename the file so its platform tag reads pyodide_<abi>_wasm32 rather than pyemscripten_<abi>_wasm32. The two spell the same ABI, but PyPI standardised on the first spelling (PEP 783) while Pyodide and FreeCAD's loader use the second. The bytes are identical; only the name changes.

Versioning and compatibility

Each file is tied to one Pyodide ABI, named by the platform tag, and to the FreeCAD build it was generated from. The guest carries a copy of the host's surface stamp; if the two disagree the sandbox still starts, but members fail to cross as they are used. A wheel is therefore only useful with a FreeCAD whose curated surface matches the one it was built against.

Where it comes from

Home: https://github.com/realthunder/FreeCAD

Built from src/App/PyodideHost/guest. The design, the runtime bootstrap and the package flow are documented in docs/PyodideHost.md and docs/Sandbox.md.