from statistics import mean, pstdev

rewards = [1.0, 0.0, 1.0, 0.0]
group_mean = mean(rewards)
group_std = pstdev(rewards)

# Equal rewards give us nothing to rank within this group.
if group_std == 0:
    advantages = [0.0] * len(rewards)
else:
    advantages = [(r - group_mean) / group_std for r in rewards]

print(advantages)
# [1.0, -1.0, 1.0, -1.0]