问题描述：
在python3.5环境中导入cv2报错，在python2.7中正常。注：命令行的前缀RL_2018HW是python3.5的环境。
(RL_2018HW) gordon@gordon-5577:$ python Python 3.5.2 (default, Nov 23 2017, 16:37:01) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import cv2 Traceback (most recent call last): File "<stdin>", line 1, in <module> ImportError: /opt/ros/kinetic/lib/python2.7/dist-packages/cv2.so: undefined symbol: PyCObject_Type gordon@gordon-5577:$ python Python 2.7.12 (default, Dec 4 2017, 14:50:18) [GCC 5.4.0 20160609] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import cv2
从报错可以看出，在python3.5虚拟环境中，程序试图从 /opt/ros/kinetic/lib/python2.7/dist-packages/ 路径下导入cv2.so库文件。然而，该路径下的库文件是属于python2.7（普通环境）的，python3.5的cv2.so文件安装在虚拟环境对应的路径之下。
(RL_2018HW) gordon@gordon-5577:$ pip show opencv-python Name: opencv-python Version: 3.4.1.15 Summary: Wrapper package for OpenCV python bindings. Home-page: https://github.com/skvark/opencv-python Author: None Author-email: None License: MIT Location: /home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages Requires: numpy Required-by: baselines
因此，虚拟环境中，程序得从 /home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages 路径下导入cv2.so文件。
解决方法：
如何修改python程序读取库文件的路径呢？
首先介绍2种方法，用于查看python读取库文件路径（以python3.5的环境为例）
1. 读取环境变量$PYTHONPATH
(RL_2018HW) gordon@gordon-5577:~$ echo $PYTHONPATH /home/gordon/python-virtual-environments/RL_2018HW/gym-gazebo:/home/gordon/ros_ws/devel/lib/python2.7/dist-packages:/opt/ros/kinetic/lib/python2.7/dist-packages:/opt/movidius/caffe/python
2. 在python控制台中查看
(RL_2018HW) gordon@gordon-5577:~$ python Python 3.5.2 (default, Nov 23 2017, 16:37:01) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import sys >>> sys.path ['', '/home/gordon/python-virtual-environments/RL_2018HW/gym-gazebo', '/home/gordon/ros_ws/devel/lib/python2.7/dist-packages', '/opt/ros/kinetic/lib/python2.7/dist-packages', '/opt/movidius/caffe/python', '/home/gordon/python-virtual-environments/RL_2018HW/lib/python35.zip', '/home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5', '/home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/plat-x86_64-linux-gnu', '/home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/lib-dynload', '/usr/lib/python3.5', '/usr/lib/python3.5/plat-x86_64-linux-gnu', '/home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages', '/home/gordon/python-virtual-environments/RL_2018HW/baselines', '/home/gordon/python-virtual-environments/RL_2018HW/pybox2d/library']
可以发现，两种方式出现了路径 /opt/ros/kinetic/lib/python2.7/dist-packages ，且其优先级比路径 /home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages 高。
我们希望程序优先读取python3.5的库路径，对应有两种方式！
1. 修改环境变量$PYTHONPATH
- 修改当前终端的环境变量
export PYTHONPATH=/home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages
- 修改当前用户的环境变量
当使用命令激活ROS时，即运行 source /opt/ros/kinetic/setup.bash ，ROS会添加 /opt/ros/kinetic/lib/python2.7/dist-packages 到python路径。因此，需要在 .bashrc 文件中注释掉该命令行，并再次source读取环境变量。
2. 在python控制台中移除错误路径
(RL_2018HW) gordon@gordon-5577:~$ python Python 3.5.2 (default, Nov 23 2017, 16:37:01) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import cv2 Traceback (most recent call last): File "<stdin>", line 1, in <module> ImportError: /opt/ros/kinetic/lib/python2.7/dist-packages/cv2.so: undefined symbol: PyCObject_Type >>> import sys >>> sys.path.remove('/opt/ros/kinetic/lib/python2.7/dist-packages') >>> import cv2