Optuna

Open-source hyperparameter optimization framework. 10K+ GitHub stars, 1.43M+ monthly downloads, used in 16K+ applications. Define-by-run API, efficient sampling (TPE, CMA-ES), pruning of unpromising trials.

Optuna v5.0 (7 September 2026) makes multivariate TPE with the constant-liar heuristic and multi-objective TPE the defaults, adds PED-ANOVA importance by default, a constraints API, and a q-batch GPSampler, and ships Rustuna, a Rust core PFN reports at up to 1,000 times faster with about half the memory at 100,000 trials; the package sees about 18.3 million downloads a month.