SyntaxError: invalid syntax for roscore

melodic

asked 2021-07-27 11:05:22 -0500

SlowAnnealing
1 ●3 ●3 ●3

updated 2021-07-27 12:15:12 -0500

jarvisschultz

9031 ●58 ●128 ●143 https://www.linkedin.c...

I tried to install relaunch roscore on Ubuntu 18.04 but got the following error. Any advice is greatly appreciated.

Traceback (most recent call last):
  File "/opt/ros/melodic/bin/roscore", line 72, in <module>
    import roslaunch
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 52, in <module>
    from . import param_dump as roslaunch_param_dump
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/param_dump.py", line 40, in <module>
    import roslaunch.config
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/config.py", line 45, in <module>
    import rospkg.distro
  File "/usr/lib/python2.7/dist-packages/rospkg/distro.py", line 44, in <module>
    import yaml
  File "/usr/lib/python3/dist-packages/yaml/__init__.py", line 284
    class YAMLObject(metaclass=YAMLObjectMetaclass):
                              ^
SyntaxError: invalid syntax

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answered 2021-07-27 16:24:55 -0500

Mike Scheutzow
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It appears you have not installed apt package python-yaml, and/or your environment variable PYTHONPATH is not right. If it has any entries for python3 in it, that going to cause you headaches.

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Thanks for your suggestion, Mike. I believe my conda environment disrupt the ROS. The following line in ~/.bashrc was commented out but the error persist even after python-yaml is confirmed installed in my machine. #export PYTHONPATH="/usr/lib/python3/dist-packages:$PYTHONPATH"

Since there is an entry for python3, the only option is reinstall ROS Melodic. Correct?

SlowAnnealing ( 2021-07-27 16:58:36 -0500 )edit

You shouldn't have to. Try deleting the PYTHONPATH variable and source setup.bash again:

unset PYTHONPATH
source /opt/ros/melodic/setup.bash

Mike Scheutzow ( 2021-07-27 17:46:48 -0500 )edit

I appreciate your expertise, Mike. Your solution works perfect without any impact on conda.

SlowAnnealing ( 2021-07-27 18:42:56 -0500 )edit

I'm happy this got resolved, but please be mindful of posting duplicates.

See #q221035, #q11479 and perhaps even #q9791.

gvdhoorn ( 2021-07-28 00:55:30 -0500 )edit

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answered 2022-02-21 12:03:12 -0500

atef
1

I you still facing the issue the solution works for me now is switching between python versions using this method (https://www.fosslinux.com/39384/switc...) then adding an alias of switching between the python versions in the basrc file alias switch_python_version='sudo update-alternatives --config python' then source source ~/.bashrc

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