With python2.7, installing ray[rllib] will fail because the following dependency chain breaks:

• rllib -> openai-gym -> atari-py ~0.2

The atari-py 0.2.3 and 0.2.2 cannot be installed in python2.7 (openai/atari-py#59)

EDIT:

On 06/20-21, openai gyms was updated to use atari-py 0.2 version, which is not compatible with python2.

FIX:

pip install ray[rllib] gym[atari]==0.15.2