这段时间搞了VINS-Mono和深度学习的结合，需要调用cv_bridge进行通信，而ROS Melodic默认安装的cv_bridge是支持python2.7的，而现在的深度学习代码都是用Python3写的，所以就出现了问题。python3中直接调用cv_bridge的话会有报错:
ImportError: dynamic module does not define module export function (PyInit_cv_bridge_boost)
不过幸运的是这个问题可以自己动手解决，cv_bridge其实是ROS的一个功能包，所以我们可以基于源码编译自己的cv_bridge。在需要的时候覆盖掉ROS自带的进行调用。
sudo apt-get install python-catkin-tools python3-dev python3-catkin-pkg-modules python3-numpy python3-yaml ros-melodic-cv-bridge
mkdir -p cv-bridge-py3.7/src
catkin config -DPYTHON_EXECUTABLE=/home/qyz/anaconda3/envs/pytorch/bin/python3 -DPYTHON_INCLUDE_DIR=/home/qyz/anaconda3/envs/pytorch/include/python3.7m -DPYTHON_LIBRARY=/home/qyz/anaconda3/envs/pytorch/lib/libpython3.7m.so -DCMAKE_BUILD_TYPE=Release -DSETUPTOOLS_DEB_LAYOUT=OFF
catkin config --install
git clone https://github.com/ros-perception/vision_opencv.git src/vision_opencv
apt-cache show ros-melodic-cv-bridge | grep Version
cd src/vision_opencv/
git checkout 1.13.0
cd ../../
catkin build cv_bridge
这里还有一点要很需要注意的是build cv_bridge会用到C++的opencv库， ../vision_opencv/cv_bridge/CMakeLists.txt中有相关的find_package命令。而安装ROS会自带OpenCV,它通常和自己默认的安装目录不一样，会出现在
/usr/include/usr/libx86_64-linux-gnu/
这时如果其他基于C++的功能包（比如VINS-Mono）和cv_bridge-py37有交互的话，就很有可能因为他们编译时所基于的C++的OpenCV版本不一致而报错。C++对于库的版本很敏感，根本比不了python中conda提供的环境隔离方便。所以最好的办法就是维护自己环境的干净和一致性，环境中的C++库都只有一个版本。
编译通过后我们就可以在需要时调用，基于ROS的工作空间覆盖机制，我们source了以后就会有限搜索新的路径，从而覆盖掉 /opt/ros/melodic/share里的cv_bridge.
source install/setup.bash --extend
打印环境变量验证
echo $ROS_PACKAGE_PATH
- source前: /opt/ros/melodic/share
- source后: /home/qyz/catkin_ws/cv-bridge-py3.7/install/share:/opt/ros/melodic/share
这时在当前环境中就可以正常使用python3解释器跑py脚本了
The source command generally overwrites one another in the sense that if you have 2 separate cakin workspaces A and B, if you perform source devel/setup.bash in workspace A and then perform source devel/setup.bash in workspace B, the second command will override the first command and the packages in workspace A will not be available to you.
To overcome this, in workspace A, perform source devel/setup.bash as usual but after this, going into workspace B ensure to perform source devel/setup.bash --extend and now packages from both workspaces will be enabled. In a similar light, perhaps you could try source /opt/ros/melodic/setup.bash and then source devel/setup.bash --extend in your workspace
大概意思就是连续的source后边的会覆盖前边的，对后一个使用extend就会保留前面的了。
参考链接
Original: https://blog.csdn.net/Robert_Q/article/details/125565487
Author: 图南i
Title: Ubuntu 18.04 ROS Melodic中调用支持Python3的cv_bridge
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