Welcome to Blackjax#

Blackjax is a library of samplers for JAX that works on CPU as well as GPU. It is designed with two categories of users in mind:

  • People who just need state-of-the-art samplers that are fast, robust and well tested;

  • Researchers who can use the library’s building blocks to design new algorithms.

It integrates really well with PPLs as long as they can provide a (potentially unnormalized) log-probability density function compatible with JAX. And while you’re here:

import jax
import jax.numpy as jnp
import jax.scipy.stats as stats
import numpy as np

import blackjax

observed = np.random.normal(10, 20, size=1_000)
def logprob_fn(x):
    logpdf = stats.norm.logpdf(observed, x["loc"], x["scale"])
    return jnp.sum(logpdf)

# Build the kernel
step_size = 1e-3
inverse_mass_matrix = jnp.array([1., 1.])
nuts = blackjax.nuts(logprob_fn, step_size, inverse_mass_matrix)

# Initialize the state
initial_position = {"loc": 1., "scale": 2.}
state = nuts.init(initial_position)

# Iterate
rng_key = jax.random.PRNGKey(0)
step = jax.jit(nuts.step)
for _ in range(1_000):
   _, rng_key = jax.random.split(rng_key)
   state, _ = step(rng_key, state)


Blackjax is written in pure Python but depends on XLA via JAX. Since the JAX installation depends on your CUDA version BlackJAX does not list JAX as a dependency. If you simply want to use JAX on CPU, install it with:

pip install jax jaxlib

Follow these instructions to install JAX with the relevant hardware acceleration support.

Then install BlackJAX

pip install blackjax

Blackjax by example