Python2 and python3 conflict is headache. This post shares about how to solve this conflict.

1. Basic Setup

• Problem on Yaml

  Traceback (most recent call last):
    File "/catkin_ws/src/ros_python3_issues/src/issue_yaml.py", line 3, in <module>
   import rospy
    File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/__init__.py", line 47, in <module>
   from std_msgs.msg import Header
    File "/opt/ros/melodic/lib/python2.7/dist-packages/std_msgs/msg/__init__.py", line 1, in <module>
   from ._Bool import *
    File "/opt/ros/melodic/lib/python2.7/dist-packages/std_msgs/msg/_Bool.py", line 5, in <module>
   import genpy
    File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/__init__.py", line 34, in <module>
   from . message import Message, SerializationError, DeserializationError, MessageException, struct_I
    File "/opt/ros/melodic/lib/python2.7/dist-packages/genpy/message.py", line 44, in <module>
   import yaml
  ModuleNotFoundError: No module named 'yaml'
  

• solve

  sudo apt-get install python3-pip python3-yaml
  sudo pip3 install rospkg catkin_pkg
  

  2. cv_bridge Issue

• If you’ll try to use cv_bridge for OpenCV you are most likely to get the following exception:

  Traceback (most recent call last):
    File "/catkin_ws/src/ros_python3_issues/src/issue_cv_bridge.py", line 23, in <module>
  ros_image = bridge.cv2_to_imgmsg(numpy.asarray(empty_image), encoding="rgb8") # convert PIL image to ROS image
    File "/opt/ros/melodic/lib/python2.7/dist-packages/cv_bridge/core.py", line 259, in cv2_to_imgmsg
  if self.cvtype_to_name[self.encoding_to_cvtype2(encoding)] != cv_type:
    File "/opt/ros/melodic/lib/python2.7/dist-packages/cv_bridge/core.py", line 91, in encoding_to_cvtype2
  from cv_bridge.boost.cv_bridge_boost import getCvType
  ImportError: dynamic module does not define module export function (PyInit_cv_bridge_boost)
  

  • The issue is that cv_bridge is built only for python 2.7 so our python 3 interpreter is trying to use cv_bridge for 2.7 and fails, lets built it for Python 3:
  • First, let’s install some tools we’ll need for the build process

sudo apt-get install python-catkin-tools python3-dev python3-numpy

Now, create new catkin_build_ws to avoid any future problems with catkin_make(assuming you are using it) and config catkin to use your python 3(3.6 in my case) when building packages:

• install in system python

  mkdir ~/catkin_build_ws && cd ~/catkin_build_ws
  catkin config -DPYTHON_EXECUTABLE=/usr/bin/python3 -DPYTHON_INCLUDE_DIR=/usr/include/python3.6m -DPYTHON_LIBRARY=/usr/lib/x86_64-linux-gnu/libpython3.6m.so
  catkin config --install
  

• under miniconda.

  mkdir ~/catkin_build_ws && cd ~/catkin_build_ws
  catkin config -DPYTHON_EXECUTABLE=/home/ran/miniconda3/bin/python3 -DPYTHON_INCLUDE_DIR=/home/ran/miniconda3/bin/python3.6m -DPYTHON_LIBRARY=/home/ran/miniconda3/lib/libpython3.6m.so
  catkin config --install
  

  clone official vision_opencv repo:

  mkdir src
  cd src
  git clone -b melodic https://github.com/ros-perception/vision_opencv.git
  

  finally, let’s build and source the package:

  cd ~/catkin_build_ws
  catkin build cv_bridge
  source install/setup.bash --extend
  

  That’s it! now you can use cv_bridge from Python 3!

  3. rostest Issue

  ```python … logging to /root/.ros/log/rostest-477a6ee2f64d-4170.log [ROSUNIT] Outputting test results to /root/.ros/test_results/ros_python3_issues/rostest-test_test_issue_rosunit.xml Traceback (most recent call last): File “/catkin_ws/src/ros_python3_issues/test/issue_rosunit.py”, line 18, in rostest.rosrun('ros_python3_issues', 'issue_rosunit', TestROSUnitIssue) File "/opt/ros/melodic/lib/python2.7/dist-packages/rostest/__init__.py", line 146, in rosrun result = rosunit.create_xml_runner(package, test_name, result_file).run(suite) File "/opt/ros/melodic/lib/python2.7/dist-packages/rosunit/xmlrunner.py", line 275, in run result.print_report(stream, time_taken, out_s, err_s) File "/opt/ros/melodic/lib/python2.7/dist-packages/rosunit/xmlrunner.py", line 202, in print_report stream.write(ET.tostring(self.xml(time_taken, out, err).getroot(), encoding='utf-8', method='xml')) TypeError: write() argument must be str, not bytes[Testcase: testissue_rosunit] ... ok [ROSTEST]-----------------------------------------------------------------------SUMMARY

• RESULT: SUCCESS
• TESTS: 0
• ERRORS: 0
• FAILURES: 0 ```

This issue was actually already solved but for some reason, it is not yet available on the apt repository so we’ll simply install it from source:

cd ~/catkin_ws/src
git clone 
https://github.com/ros/roscd ..
catkin_make_isolated --install --pkg rosunit -DCMAKE_BUILD_TYPE=Release --install-space /opt/ros/melodic

That’s it!