This is a Python library used to train an open source quadruped robot platform: Rex. Rex is a 12 joints robot with 3 motors for each leg (Shoulder, Leg and Foot). The platform is an open source 3D printable design made by Deok-yeon Kim and available on Thingiverse.The robot model is imported in pyBullet creating an URDF file while the servo motors are modeled with the motor.py class.This library uses the Proximal Policy Optimization algorithm with a hybrid policy that can be varied continuously from fully user-specified to entirely learned from scratch. A twofold approach is used to implement the Rex Gym Environments: "inverse kinematics controller" and "open loop". The first approach is based on a Bezier open-loop controller meanwhile The "open loop" mode consists, in some cases, in let the system lean from scratch and others providing a simple trajectory.