What are the best RL libraries for Python?

The best Python libraries for reinforcement learning (RL) are OpenAI Gym, Stable Baselines3, and Ray RLlib. These tools provide frameworks for building, training, and testing RL agents, with varying focuses on flexibility, ease of use, and scalability. Each library serves different needs, from prototyping to deploying large-scale RL systems, and all are widely adopted in both research and industry.

OpenAI Gym is a foundational library for RL experimentation. It offers a standardized interface to over 100 pre-built environments, such as classic control tasks (e.g., CartPole), Atari games, and robotics simulations. Developers can quickly test algorithms by interacting with environments through a simple API—calling env.step(action) to take an action and receive observations, rewards, and termination signals. For example, training an agent to balance a pole in the CartPole environment requires just a few lines of code to set up the environment and loop through episodes. While Gym itself doesn’t implement algorithms, its compatibility with other libraries (like TensorFlow or PyTorch) makes it a staple for benchmarking RL models.

Stable Baselines3 builds on Gym’s foundation by providing high-quality implementations of popular RL algorithms like PPO, DQN, and SAC. It focuses on usability, offering a consistent API for training and evaluating agents. For instance, training a Proximal Policy Optimization (PPO) agent on the MountainCar environment can be done in under 10 lines of code, with built-in support for saving models, logging metrics, and hyperparameter tuning. The library is designed for reliability—it’s rigorously tested and actively maintained—making it ideal for developers who want to avoid reinventing the wheel. Its integration with PyTorch also allows customization of neural network architectures, catering to advanced users.

Ray RLlib excels in scalability for complex or distributed RL workloads. Built on the Ray framework, it supports multi-agent setups, hyperparameter optimization, and large-scale parallel training across clusters. For example, training a multi-agent system where robots collaborate in a warehouse simulation can leverage RLlib’s distributed computing capabilities to speed up experimentation. Companies like Amazon and Anthem use RLlib for real-world applications due to its production-ready features, such as fault tolerance and Kubernetes integration. While it has a steeper learning curve than Gym or Stable Baselines3, its flexibility and performance make it a top choice for enterprise-level RL projects. Other notable libraries include Dopamine (focused on reproducibility) and Tianshou (modular design), but the three above are the most versatile for most use cases.