[Proposal] Make pygame dependency optional for classic control

[araffin]

Proposal

If I understand well, pygame is only used for rendering, so it should be possible to install gym and use classic control environments without installing pygame.

Motivation

Fewer dependencies are always nice.
Classic control envs are usually used to debug things/try things quickly without necessary the need to render them.
This would also prevent unexpected/inconsistent behavior, as in #2634 #2641 or #2642 (where previously a pip install gym would have sufficed).

Pitch

Make pygame dependency optional, throw an error if the user wants to render and pygame is not installed.

Alternatives

Do nothing.

Checklist

• I have checked that there is no similar issue in the repo (required)

Tasks

Beta Give feedback

No tasks being tracked yet.

[jkterry1]

But pygame already basically is an optional dependency for classic control? pip install gym installs all the code in gym, including the classic control envs. pip install gym[classic_control] only adds the pygame dependency.

See

gym/setup.py

Line 16 in 57b157b

"classic_control": ["pygame==2.1.0"],

[araffin]

But pygame already basically is an optional dependency for classic control?

# install gym 0.23.1
pip install git+https://github.com/openai/gym/@850247f888cc4642561488b7c17f8b29873b31bd

import gym

gym.make("Pendulum-v1")

yields ModuleNotFoundError: No module named 'pygame' as noted by other issues, hence you cannot use classic control env without pygame, even when you don't want to render anything.

with gym 0.21:

pip install gym==0.21

import gym

gym.make("Pendulum-v1")

does not throw any error and you can use the env without rendering anything.

[RedTachyon]

Yea, this absolutely makes sense. It should be enough to just move the pygame imports into the body of render

[jkterry1]

Closing in favor of #2712