Installation Problems
Each entry is a symptom, its cause, and a solution.
ModuleNotFoundError: No module named 'menos'
| Symptom | import menos fails |
| Cause | MENOS is not installed in the Python environment you are running, most often because the virtual environment is not the one in use |
| Solution | Run the interpreter from the environment you installed into (.venv/bin/python), or activate it. Check with python -c "import sys; print(sys.executable)" |
ImportError mentioning _core
| Symptom | import menos fails with an ImportError about menos._core |
| Cause | The native extension did not build, or was built for a different Python version than the one importing it |
| Solution | Reinstall into the environment you import from (for example uv venv .venv --python 3.14). An extension built for one Python version cannot load in another. On Windows, DLL load failed means libopenblas.dll is missing next to _core*.pyd in the installed menos package; reinstall after running scripts\fetch-openblas-windows.ps1 |
ModuleNotFoundError: No module named 'qiskit'
| Symptom | from menos.qiskit import MENOSBackend fails |
| Cause | Qiskit is an optional extra |
| Solution | Install with the extra: pip install "menos[qiskit]" or uv add "menos[qiskit]" (from a source checkout: uv pip install --python .venv/bin/python ".[qiskit]"). The native API works without it |
The installer says the package requires a different Python
| Symptom | pip or uv refuses to install, naming a Python version requirement |
| Cause | menos requires Python >=3.10, and PyPI has wheels only for CPython 3.10–3.14 on Linux x86-64 and Windows x64 |
| Solution | Use Python 3.10–3.14 (uv python install 3.14 installs one). No source distribution is published; on other platforms install from source |
Qiskit version problems
| Symptom | MENOSBackend or MENOSEstimator raise errors from inside Qiskit, or behave unexpectedly |
| Cause | The qiskit extra pins 2.5.2. A different installed version is untested |
| Solution | uv pip install --python .venv/bin/python "qiskit==2.5.2" |
The build cannot download its dependencies
| Symptom | The build fails while configuring, with a network or git error |
| Cause | When building from source, CMake fetches GoogleTest and nlohmann/json from GitHub at configure time, even for a Python install. They are used only by the test suite, but the build needs them |
| Solution | Ensure git is installed and github.com is reachable (check proxy settings). Offline builds are not supported |
A stale build after changing options
| Symptom | A rebuild ignores a new CMake option, for example turning CUDA on |
| Cause | The previous configuration is cached in the build directory (build/py-… for Python installs) |
| Solution | Delete that build directory and install again |
Installation succeeded but examples fail
| Symptom | An example in the repository fails immediately |
| Cause | Usually the qiskit extra is missing, or you are running a different interpreter |
| Solution | Run Verify Installation |