Build fails with -DUSE_EMBEDDED_PYTHON=On

[planepaper]

Environment

• OS: Ubuntu 24.04
• Compiler: GCC 14.2.0
• CMake: 3.28.3
• Branch/commit: master/c5a063d

Context

There are example scenes (e.g. under data/Scenes/AnimatedBody_2D.json) that use embedded
Python scripts to animate rigid bodies dynamically. These scenes require
-DUSE_EMBEDDED_PYTHON=On to function. However, this option cannot be
built successfully, making these examples completely unusable.

I also checked out the commit(tag:2.10.0) where USE_EMBEDDED_PYTHON support was
originally introduced, and the build failed there as well with the same
errors — so this feature appears to have never been buildable.

Steps to Reproduce

cmake -DCMAKE_BUILD_TYPE=Release -DUSE_PYTHON_BINDINGS=On -DUSE_EMBEDDED_PYTHON=On ..

Error

CMake configuration fails with:
The dependency target "pysplishsplash_embed" of target "SimulatorBase" does not exist.

This happens because add_subdirectory(Simulator) is processed before
add_subdirectory(pySPlisHSPlasH) in the top-level CMakeLists.txt,
so pysplishsplash_embed does not exist yet when Simulator/CMakeLists.txt
adds it to SIMULATION_DEPENDENCIES.

After commenting out pysplishsplash_embed from SIMULATION_DEPENDENCIES,
the build progresses but hits a second error:
Target "SimulatorBase" links to: pybind11::embed but the target was not found.

pySPlisHSPlasH/CMakeLists.txt references pybind11::embed, but the
bundled extern/pybind/CMakeLists.txt only defines pybind11::headers
— pybind11::embed does not exist.

My Understanding (may be incorrect)

It seems like there is a circular dependency between SimulatorBase and
pysplishsplash_embed:

• Simulator/CMakeLists.txt links SimulatorBase to pysplishsplash_embed
• pySPlisHSPlasH/CMakeLists.txt links pysplishsplash_embed to SimulatorBase

Combined with the subdirectory ordering and the missing pybind11::embed
target, building with -DUSE_EMBEDDED_PYTHON=On does not seem possible
with the current CMake configuration.

I may be misunderstanding the intended build setup, so any guidance on
how to correctly build with this option would be appreciated.

Expected Behavior

-DUSE_EMBEDDED_PYTHON=On should configure and build successfully.

[janbender]

Did you set the CMake option Python_EXECUTABLE correctly?

[planepaper]

Thanks for the response. Yes, Python_EXECUTABLE was set correctly.

Update: the Python wrapping (wheel) build works fine on its own, and I confirmed that rigid body manipulation through the Python API works as expected. So for anyone wanting Python-driven rigid body control per step, using the Python package is the recommended path — the interpreter is already running when you import splishsplash, and the necessary APIs are exposed.

The embedded Python build (USE_EMBEDDED_PYTHON=On) still fails due to a circular dependency: SimulatorBase's ScriptObject needs Embedded.cpp/h (currently in pySPlisHSPlasH/), but pysplishsplash_embed itself depends on SimulatorBase. CMake cannot resolve this cycle.

A possible fix would be to move Embedded.cpp/h and the binding *Module.cpp files into SimulatorBase when USE_EMBEDDED_PYTHON=On, and drop pysplishsplash_embed. This is the minimal change but compiles the binding sources twice (once for embedded, once for the wheel).

Since the Python wrapping covers my use case, I don't have an urgent need to fix the embedded build right now — leaving this as a reference in case it's useful.

Thanks again.