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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。

1. (RL_2018HW) gordon@gordon-5577:$ python
2. Python [b]3.5.2[/b] (default, Nov 23 2017, 16:37:01)
3. [GCC 5.4.0 20160609] on linux
4. Type "help", "copyright", "credits" or "license" for more information.
5. >>> import cv2
6. Traceback (most recent call last):
7. File "<stdin>", line 1, in <module>
8. ImportError: /opt/ros/kinetic/lib/python2.7/dist-packages/cv2.so: undefined symbol: PyCObject_Type
10. gordon@gordon-5577:$ python
11. Python [b]2.7.12[/b] (default, Dec 4 2017, 14:50:18)
12. [GCC 5.4.0 20160609] on linux2
13. Type "help", "copyright", "credits" or "license" for more information.
14. >>> import cv2

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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。

1. (RL_2018HW) gordon@gordon-5577:$ pip show opencv-python
2. Name: opencv-python
3. Version: 3.4.1.15
4. Summary: Wrapper package for OpenCV python bindings.
5. Home-page: https://github.com/skvark/opencv-python
6. Author: None
7. Author-email: None
8. License: MIT
9. Location: /home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages
10. Requires: numpy
11. Required-by: baselines

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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

1. (RL_2018HW) gordon@gordon-5577:~$ echo $PYTHONPATH
2. /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

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2. 在python控制台中查看

1. (RL_2018HW) gordon@gordon-5577:~$ python
2. Python 3.5.2 (default, Nov 23 2017, 16:37:01)
3. [GCC 5.4.0 20160609] on linux
4. Type "help", "copyright", "credits" or "license" for more information.
5. >>> import sys
6. >>> sys.path
7. ["", "/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"]

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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

1. export PYTHONPATH=/home/gordon/python-virtual-environments/RL_2018HW/lib/python3.5/site-packages

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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

1. (RL_2018HW) gordon@gordon-5577:~$ python
2. Python 3.5.2 (default, Nov 23 2017, 16:37:01)
3. [GCC 5.4.0 20160609] on linux
4. Type "help", "copyright", "credits" or "license" for more information.
5. >>> import cv2
6. Traceback (most recent call last):
7.   File "<stdin>", line 1, in <module>
8. ImportError: /opt/ros/kinetic/lib/python2.7/dist-packages/cv2.so: undefined symbol: PyCObject_Type
9. >>> import sys
10. >>> sys.path.remove("/opt/ros/kinetic/lib/python2.7/dist-packages")
11. >>> import cv2

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