Zero Division Error on reset with pybullet

[araffin]

Hello,

I'm facing a weird issue with "HalfCheetahBulletEnv-v0", it throws a zero division error on reset (see traceback below).

I've tried different version of pybullet (I'm using the latest one) but I could not reproduce the error in a docker image :/

System: Ubuntu 20.04
Python 3.7.9
Cpu only

Even weirder, the error does not occur when using "Ant" or when running the code in an interactive python shell...

I traced down the issue to

ROBOT PART 'link0_6'
ROBOT JOINT 'bthigh'
bthigh 0 4

where upper_limit = lower_limit = 0 for that joint...

../../../../miniconda3/lib/python3.7/site-packages/gym/core.py:237: in reset
    return self.env.reset(**kwargs)
utils/wrappers.py:169: in reset
    return self.env.reset()
../../../../miniconda3/lib/python3.7/site-packages/gym/wrappers/time_limit.py:25: in reset
    return self.env.reset(**kwargs)
../../../../miniconda3/lib/python3.7/site-packages/pybullet_envs/gym_locomotion_envs.py:32: in reset
    r = MJCFBaseBulletEnv.reset(self)
../../../../miniconda3/lib/python3.7/site-packages/pybullet_envs/env_bases.py:87: in reset
    s = self.robot.reset(self._p)
../../../../miniconda3/lib/python3.7/site-packages/pybullet_envs/robot_bases.py:144: in reset
    s = self.calc_state(
../../../../miniconda3/lib/python3.7/site-packages/pybullet_envs/robot_locomotors.py:36: in calc_state
    j = np.array([j.current_relative_position() for j in self.ordered_joints],
../../../../miniconda3/lib/python3.7/site-packages/pybullet_envs/robot_locomotors.py:36: in <listcomp>
    j = np.array([j.current_relative_position() for j in self.ordered_joints],
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <robot_bases.Joint object at 0x7f80289b61d0>

    def current_relative_position(self):
      pos, vel = self.get_state()
      pos_mid = 0.5 * (self.lowerLimit + self.upperLimit)
>     return (2 * (pos - pos_mid) / (self.upperLimit - self.lowerLimit), 0.1 * vel)
E     ZeroDivisionError: float division by zero

../../../../miniconda3/lib/python3.7/site-packages/pybullet_envs/robot_bases.py:351: ZeroDivisionError

Do you have any idea of what may cause this?

[araffin]

After further investigation, it seems to comes from matplotlib

this code work:

import gym
import pybullet_envs

env = gym.make("HalfCheetahBulletEnv-v0")
print(env.reset())

but this one does not:

import gym
import pybullet_envs

env = gym.make("HalfCheetahBulletEnv-v0")

from matplotlib import pyplot as plt
print(env.reset())

[araffin]

works with matplotlib==3.0.3
but does not work with matplotlib >= 3.1.0

[araffin]

solved the issue by adding

import matplotlib
matplotlib.use("TkAgg")

at the top instead of GTK3Agg