解决python3.5无法导入cv2.so的问题

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