ROS 執行launch報錯 python2.7錯誤

mjm@mjm-pc:~/Desktop/ros_robot_pro$ roslaunch ros_urdf_PK test.launch 
Traceback (most recent call last):
  File "/opt/ros/melodic/bin/roslaunch", line 34, in 
    import roslaunch
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 57, in 
    from .launch import ROSLaunchRunner
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/launch.py", line 55, in 
    from roslaunch.nodeprocess import create_master_process, create_node_process, DEFAULT_TIMEOUT_SIGINT, DEFAULT_TIMEOUT_SIGTERM
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roslaunch/nodeprocess.py", line 53, in 
    from rosmaster.master_api import NUM_WORKERS
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/__init__.py", line 35, in 
    from .main import rosmaster_main
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/main.py", line 43, in 
    import rosmaster.master
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/master.py", line 47, in 
    import rosmaster.master_api
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/master_api.py", line 72, in 
    from rosmaster.util import xmlrpcapi
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rosmaster/util.py", line 48, in 
    from defusedxml.xmlrpc import monkey_patch
ModuleNotFoundError: No module named 'defusedxml'

解决办法：切换到 python-2.7

那说明Python 的替代版本尚未被 update-alternatives 命令识别.想解决这个问题,

我们需要更新一下替代列表,将python2.7和python3.4放入其中,使用如下命令将python2.7和python3.5加入到python列表中:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2

使用update-alternatives 来为整个系统更改Python默认版本

1、列出python版本号sudo update-alternatives --list python

２1、配置默认解释器s udo update-alternatives --config python

下面我选择编号1,选择python2.7 验证在终端输入python即可：