I’m working with a custom gym environment and running into a weird issue. I have a Python file called game.py that looks something like this:
import gym
def start_game():
environment = gym.make('AdvancedPinball-v0')
When I import this function from another file, everything works perfectly:
from game import start_game
start_game()
But when I try to create the environment directly in a new file:
import gym
environment = gym.make('AdvancedPinball-v0')
I get an error saying the custom environment can’t be found. This is really confusing because both approaches should be doing the same thing. The gym import is identical in both cases. Has anyone else encountered this problem with custom environments? What could be causing this difference in behavior?
This happens because your custom environment registration is probably in game.py or gets imported when you run start_game. Custom gym environments need to be registered before you can use gym.make() to create them. When you call start_game, it runs the whole game.py file, which registers your AdvancedPinball-v0 environment. But when you make a new file and just import gym, that registration code never runs - so gym doesn’t know your custom environment exists. You need a gym.register() call in game.py or a module it imports. Make sure that same registration happens in your new file by importing the registration module or running the registration code directly.
totally get it! it’s all about the import timing. your custom environment’s registration is tied to game.py and only kicks in during start_game. when you just import gym directly, it skips over that. just add import game at the start of your new file to fix it.
This happens because of how Python handles module imports and execution order. Your custom environment registration runs as a side effect when game.py gets imported or executed. When you call start_game(), Python loads the entire game.py module, which triggers any registration code at the module level or in imported dependencies. But when you create a standalone file that only imports gym, none of your custom environment’s registration code runs. The environment registration is missing from the namespace. I’ve hit this exact problem when refactoring gym environments into separate modules. To fix it, make sure the registration happens explicitly. Either import the module containing your environment registration before calling gym.make(), or move the registration code to a separate module that gets imported consistently across all your files.