Gym is an open source Python library developed by OpenAI that provides a toolkit for developing and comparing reinforcement learning algorithms. The library establishes a standard API for communication between learning algorithms and simulation environments, along with a comprehensive suite of pre-built environments that comply with this API. Since its release, Gym's API has become the field standard for reinforcement learning research and development, making it a foundational tool in the machine learning community.
The core purpose of Gym is to standardize how reinforcement learning algorithms interact with training environments. Rather than requiring researchers to build custom interfaces for each new environment, Gym provides a unified Python API where environments are implemented as simple classes. Users can create environment instances and interact with them through a consistent interface, as demonstrated in the library's documentation with examples like the CartPole-v1 environment. This standardization dramatically reduces friction when implementing, testing, and comparing different reinforcement learning algorithms.
Gym supports Python versions 3.7 through 3.10 on Linux and macOS, with community contributions accepted for Windows support though it is not officially supported. Installation is straightforward through pip, with the base library available via pip install gym. The library offers optional dependency groups for different environment families, allowing users to install only what they need or use pip install gym[all] for complete functionality. MuJoCo environments, a particularly important category for physics-based simulations, have transitioned from the mujoco-py dependency to the mujoco package in recent versions, with older versions remaining available but unmaintained.
A critical aspect of Gym's design is its strict environment versioning system. All environments include version suffixes like _v0, _v1, and so forth. When changes are made to environments that could impact learning results, the version number increments, ensuring reproducibility and preventing confusion when comparing experimental results across different versions.
However, the repository is currently in a transitional phase. The team maintaining Gym since 2021 has moved all future development to Gymnasium, a drop-in replacement available at the Farama Foundation. The README explicitly states that Gym will not receive future updates, and users are encouraged to migrate to Gymnasium, which maintains API compatibility through the import statement import gymnasium as gym. This transition reflects the evolution of the reinforcement learning ecosystem and the community's decision to consolidate development efforts under the Farama Foundation's stewardship.
