python - Record OpenAI gym Video with Monitor

Question

I want to record a video of my rollouts of OpenAIs gym. I use the Monitor class, but other solutions are also appreciated. This is a minimal example I created, that runs without exceptions or warnings:

import gym
from gym.wrappers import Monitor
env = Monitor(gym.make('CartPole-v0'), './video', force=True)
state = env.reset()
done = False
while not done:
    action = env.action_space.sample()
    state_next, reward, done, info = env.step(action)
env.close()

This saves a video and some metadata to the './video' folder. However, the mp4-file that is saved there has always just a size of 262 byte and the "file contains no playable steams". So it seems it is a 'video' but made up of 0 frames? I don't understand what the problem is and how to debug from here.

Answer 1

It was actually a bug that is fixed here