rosrun still use python2.7 instead of 3.8 inside noetic

asked 2020-09-21 02:56:06 -0600

nadim.arubai
37 ●2 ●3 ●6

updated 2021-04-23 21:13:40 -0600

10937 ●203 ●339 ●330 http://www.linkedin.co...

I want to use rosrun or roslaunch to run some python node in ros noetic. Although the examples I tested worked in kinetic under ubuntu 16.04, they no longer work in noetic under ubuntu 20.04. The main problem that rosrun failed to locate yaml module, while calling the python script directly work. After some checking, and trying small examples, I found the problem by putting

import sys
print(sys.version)
import yaml

by running the script with rosrun I got:

nnn@aaa:~/ros/catkin_ws/src/node_example-main/src$ rosrun node_example pytalker.py

2.7.18rc1 (default, Apr  7 2020, 12:05:55) 
[GCC 9.3.0]

by running it with default system python:

nnn@aaa:~/ros/catkin_ws/src/node_example-main/src$ python3 pytalker.py

3.8.2 (default, Jul 16 2020, 14:00:26) 
[GCC 9.3.0]

I tried many solution but all of them have failed.

for reference I used this example from ROS: https://github.com/tdenewiler/node_ex...

and "catkin_make install" build successfully with out errors or warnings.

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2

answered 2020-09-21 11:48:24 -0600

sloretz
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It looks like the problem is the example is using install(PROGRAMS ... for Python scripts, and the Python scripts have shebangs that say #!/usr/bin/env python. When the scripts are installed the shebangs are unchanged, so it uses whatever version of python the python command starts.

To fix this, the code should be using catkin_install_python(...) which rewrites the shebang's to point to the correct version (Python 2 on ROS Melodic and earlier, Python 3 on ROS Noetic).