I’m trying to figure out how to change specific parameters in OpenAI Gym environments.
For simple environments like CartPole, I can easily modify values in the __init__ method of the class. However, when dealing with Box2D-based environments, the process seems more complex.
Take the BipedalWalker environment as an example. I want to adjust parameters like MOTOR_TORQUE and LEG_VELOCITY constants. These seem to be defined differently than in basic environments.
class CustomBipedalWalker(gym.Env):
def __init__(self):
# How do I modify these constants?
self.MOTOR_TORQUE = 80 # default value
self.LEG_VELOCITY = 10 # default value
def create_robot(self):
# Robot creation with custom parameters
robot_body = self.world.CreateDynamicBody(
position=(0, ROBOT_HEIGHT),
fixtures=robot_fixture
)
return robot_body
What’s the proper way to override these Box2D environment constants? Should I inherit from the original class or is there a different approach?
I’ve hit this exact problem with BipedalWalker modifications. Box2D environments define constants at the module level, not just in the class. You’ll need to subclass the environment and override the constants in your custom constructor before calling the parent’s init.
For BipedalWalker, inherit from it and set your values like self.MOTOR_TORQUE and self.LEG_VELOCITY in init. Make sure these get used when the physics bodies are created.
Timing’s crucial here - some constants get used during world creation. Set your custom values before calling super().init() or the parent’s reset method where the world gets built.
I’d recommend checking the source code to see exactly where each constant gets referenced during setup. Really helps understand the flow.
you can also monkey patch the constants directly in the module before importing the env. i did this for lunar lander and it worked perfectly. just import gym.envs.box2d.bipedal_walker as bw then bw.MOTOR_TORQUE = 120 before creating the env instance. it’s hacky but saves you from subclassing if you don’t need other customizations.
Here’s another approach that works great for Box2D environments: create a wrapper class that modifies the environment after you instantiate it. I’ve used this with several Box2D environments including BipedalWalker. Make a wrapper that takes the base environment and dynamically updates the constants through attribute assignment. The key insight? Many Box2D constants can be modified even after the environment exists - just do it before calling reset(). For BipedalWalker specifically, you can modify the underlying environment object’s attributes directly, then call reset to rebuild the world with your new parameters. This keeps your code cleaner than subclassing and more reliable than monkey patching since you’re working with a specific instance rather than global module state. I found this super useful when running parameter sweeps for research. You can modify the same environment instance with different values without recreating everything from scratch.