pyjags 2.3.1

Python interface to JAGS for Bayesian analysis via MCMC

PyJAGS provides a Python interface to JAGS (Just Another Gibbs Sampler), a mature engine for Bayesian analysis via Markov Chain Monte Carlo (MCMC) simulation.

Why PyJAGS?

No compilation step. JAGS interprets models at runtime. Change your model code and re-run immediately, with no C++ compiler toolchain required. Even state-of-the-art HMC packages can take 30-60+ seconds to compile each model; JAGS models are ready in milliseconds.

Handles models that gradient-based samplers cannot. JAGS works with non-differentiable likelihoods, discrete parameters, mixture models, and change-point models that challenge HMC/NUTS. No divergence diagnostics to fight.

Incremental sampling is built in. JAGS retains chain state between sample() calls, so extending a run is a single line of code -- a capability that is unique among Python Bayesian packages.

Familiar model syntax. JAGS uses the BUGS language, the lingua franca of Bayesian modeling in the R ecosystem (WinBUGS, OpenBUGS, nimble). R users migrating to Python can use their existing models unchanged.

Lightweight. PyJAGS depends only on numpy, arviz, and h5py. Install with pip install pyjags and pre-built wheels for Linux and macOS.

Features

PyJAGS adds the following features on top of JAGS:

• Multicore support for parallel simulation of multiple Markov chains
• Built-in ArviZ integration via pyjags.from_pyjags() for diagnostics and visualization
• Reproducible sampling via a single seed parameter
• Generator-based sampling with iter_sample() for live convergence monitoring
• Standalone model syntax validation with check_model()
• Incremental sampling with automatic convergence detection (ESS and R-hat criteria)
• Saving and restoring MCMC sample chains to/from HDF5 files
• Merging samples along iterations or across chains for resumed sampling
• PEP 561 py.typed marker for IDE and type checker support

License: GPLv2

Compatibility

Component	Supported Versions
Python	3.12+
NumPy	1.x and 2.x
ArviZ	1.0+
macOS	Intel and Apple Silicon (M1/M2/M3/M4)
Linux	Debian/Ubuntu (tested), other distributions (untested)

Note: Python 3.10 and 3.11 were supported in earlier releases but are no longer supported because ArviZ 1.0 — a core dependency — requires Python 3.12+.

Installation

Prerequisites

A working JAGS installation, a C++ compiler, and CMake are required. PyJAGS uses CMake with pkg-config to locate the JAGS headers and libraries at build time.

macOS

1. Install JAGS via Homebrew

Apple Silicon (M1/M2/M3/M4):

Homebrew installs to /opt/homebrew on Apple Silicon Macs:

# Install Homebrew if needed (https://brew.sh)
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

# Follow Homebrew's instructions to add it to your PATH, then:
brew install jags

Make sure pkg-config can find the JAGS installation by adding this to your shell profile (~/.zprofile or ~/.zshrc):

export PKG_CONFIG_PATH="/opt/homebrew/lib/pkgconfig:$PKG_CONFIG_PATH"

Intel Mac:

Homebrew installs to /usr/local on Intel Macs and pkg-config typically finds JAGS automatically:

brew install jags

2. Set up Python and install PyJAGS

We recommend using uv to manage Python installations and virtual environments:

# Install uv if needed
curl -LsSf https://astral.sh/uv/install.sh | sh

# Install Python 3.12 and create a virtual environment
uv python install 3.12
uv venv --python 3.12 .venv
source .venv/bin/activate

# Install PyJAGS
uv pip install pyjags

To install from source (for development):

source .venv/bin/activate

git clone https://github.com/michaelnowotny/pyjags.git
cd pyjags

uv pip install -e .

Linux

Install JAGS using your distribution's package manager:

# Debian/Ubuntu
sudo apt-get install jags pkg-config

# Fedora/RHEL (untested — package names may differ)
sudo dnf install jags jags-devel pkgconf

Then install PyJAGS:

pip install pyjags

Or from source:

git clone https://github.com/michaelnowotny/pyjags.git
cd pyjags

pip install -e .

Windows

PyJAGS is not natively supported on Windows. Windows users should use WSL2 (Windows Subsystem for Linux) with an Ubuntu distribution, then follow the Linux installation instructions above.

Notebooks

The notebooks/ directory contains Jupyter notebooks demonstrating PyJAGS features:

Notebook	Description
Getting Started	Beginner-friendly introduction to Bayesian inference with PyJAGS
New in v2.3.0	Showcase of all v2.3.0 features: iter_sample(), seed=, check_model(), and more
Trading Cost Estimation	Bayesian estimation of bid-ask spreads using Hasbrouck's model (with and without a market factor)
Logistic Regression	Bayesian logistic regression with MCMC diagnostics
Eight Schools	Classic hierarchical model with prior/posterior analysis, warmup splitting, and LOO
Advanced Functionality	Parallel chains, HDF5 persistence, chain merging, and incremental sampling until convergence

If you have a native installation (macOS or Linux with JAGS and .venv set up as described above), you can run Jupyter Lab directly without Docker:

./scripts/jagslab lab

On first run, this installs notebook dependencies (JupyterLab, seaborn, scikit-learn) into your .venv and registers a "Python 3.12 (pyjags)" Jupyter kernel that points to the correct Python environment. The notebooks are pre-configured to use this kernel.

ArviZ Integration

PyJAGS includes a built-in converter for ArviZ 1.0+. Use pyjags.from_pyjags() to convert sample dictionaries returned by Model.sample() into ArviZ DataTree objects for diagnostics and visualization:

import pyjags
import arviz as az

model = pyjags.Model(code=model_code, data=data, chains=4)
model.sample(1000, vars=[])                     # burn-in
samples = model.sample(5000, vars=['mu', 'sigma'])

idata = pyjags.from_pyjags(samples)             # -> xarray.DataTree
az.summary(idata)
az.plot_trace(idata)

The converter also supports prior samples, log-likelihood extraction, and warmup splitting. See the Eight Schools notebook for a complete example.

Development Environment (jagslab)

PyJAGS also includes a Docker-based development environment managed by the jagslab CLI script. This provides a reproducible setup with JAGS, Python 3.12, and Jupyter Lab without requiring a local JAGS installation.

Quick Start (Docker)

# 1. Copy the environment configuration
cp .env.example .env          # Edit .env to customize (e.g., Jupyter port)

# 2. Build the Docker image
./scripts/jagslab build

# 3. Run the test suite
./scripts/jagslab test

# 4. Start Jupyter Lab
./scripts/jagslab start       # Opens at http://localhost:8888

jagslab Commands

Command	Description
start	Start Jupyter Lab via Docker (auto-starts container and installs pyjags)
lab	Start Jupyter Lab natively from .venv (no Docker required)
stop	Stop the Docker container
test [args]	Run the test suite
install	Install/reinstall pyjags (recompiles C++ extension)
shell	Open a bash shell in the container
python [args]	Start Python REPL or run a script in the container
build	Build the Docker image
rebuild	Rebuild image from scratch (no Docker cache)
status	Show container status
logs	Tail container logs
clean	Remove build artifacts
version	Show pyjags, JAGS, Python, and numpy versions

Running Tests

./scripts/jagslab test                                           # All tests
./scripts/jagslab test test.test_model                           # One module
./scripts/jagslab test test.test_model.TestModel.test_samples_shape  # Single test

Pre-commit Hooks

PyJAGS uses pre-commit to run ruff linting and formatting checks before each commit, preventing CI failures:

pip install pre-commit
pre-commit install

After installation, ruff will automatically check and fix files in src/ and test/ on every git commit. To run manually against all files:

pre-commit run --all-files

Working with the C++ Extension

PyJAGS includes a C++ extension (src/pyjags/console.cc) that wraps the JAGS library using pybind11. When editing Python files under src/pyjags/, changes take effect immediately thanks to the editable install. However, after editing src/pyjags/console.cc, you must recompile:

./scripts/jagslab install

Configuration

The .env file (created from .env.example) controls environment settings:

• JAGSLAB_PORT — Host port for Jupyter Lab (default: 8888)

If .env does not exist when you run a jagslab command, it is automatically created from .env.example.

Troubleshooting

macOS: symbol not found '_JAGS_NA' or missing JAGS symbols at runtime

This usually means the compiled extension was not linked against libjags. Verify that pkg-config finds your JAGS installation:

pkg-config --libs --cflags jags

If this fails or points to the wrong location, set PKG_CONFIG_PATH as described in the installation instructions above, then reinstall PyJAGS.

macOS: found architecture 'x86_64', required architecture 'arm64'

This occurs on Apple Silicon Macs when JAGS was installed via an Intel (Rosetta) Homebrew at /usr/local but Python is running natively as ARM64. The fix is to install JAGS using the native ARM Homebrew at /opt/homebrew:

# Check which Homebrew you are using
which brew
# /opt/homebrew/bin/brew  -> ARM (correct for Apple Silicon)
# /usr/local/bin/brew     -> Intel/Rosetta (will cause architecture mismatch)

# If needed, install ARM Homebrew and then:
/opt/homebrew/bin/brew install jags

Make sure PKG_CONFIG_PATH points to the ARM Homebrew pkgconfig directory (/opt/homebrew/lib/pkgconfig) so that the build picks up the correct library.

CMake errors during build

PyJAGS uses CMake (via scikit-build-core) to find and link against JAGS. If the build fails with CMake errors, ensure CMake is installed:

# macOS
brew install cmake

# Debian/Ubuntu
sudo apt-get install cmake

# pip
pip install cmake

Useful Links

• Package on the Python Package Index https://pypi.python.org/pypi/pyjags
• Project page on github https://github.com/michaelnowotny/pyjags
• JAGS manual and examples http://sourceforge.net/projects/mcmc-jags/files/

Acknowledgements

• JAGS was created by Martyn Plummer
• PyJAGS was originally created by Tomasz Miasko
• As of May 2020, PyJAGS is developed by Michael Nowotny
• Max Schulist (PR #1) proposed migrating to scikit-build-core, pyproject.toml, and pytest, ideas that inspired the packaging modernization in version 2.1
• Scout Jarman (PR #2) independently proposed CI/CD workflows, CMake builds, and JAGS install scripts, reinforcing the direction of the modernization effort