MCMC sampling hangs in tests/test_save_load.py::test_save_load_vi_mcmc possibly after external dependency updates
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- Python
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Description
Recently the test tests/test_save_load.py::test_save_load_vi_mcmc started hanging during PyMC’s NUTS initialization and sampling. The model and ONNX/JAX initialization succeed, but sampling does not progress (even with single-threaded execution).
Observed logs:
Model initialized successfully
ONNX/JAX backend fallback messages (TPU/GPU not available, falling back to CPU)
PyMC: “Initializing NUTS using adapt_diag...”
Multiprocess sampling (2 chains in 2 jobs)
Progress bar stuck at 0 draws
What changed:
This test previously passed; the issue seems to have appeared after updating dependencies (PyMC, JAX, ONNX, etc.).
Run pytest tests/test_save_load.py::test_save_load_vi_mcmc to reproduce. Observe hang during MCMC sampling.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running pytest tests/test_save_load.py::test_save_load_vi_mcmc and inspect the hang during PyMC NUTS initialization and sampling. Compare the dependency updates with the previously passing GitHub Actions run, focusing on the PyMC, JAX, and ONNX-related environment described in the issue. Done means the test completes successfully without hanging.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- testing
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100