$ roscore
结果

Traceback (most recent call last):
File “/opt/ros/melodic/bin/roscore”, line 36, 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’

主动更新了 python 版本导致 ros 环境引导的位置出现了问题。
解决方法：

• 删除原有的 python 软连接
  cd /usr/bin/
sudo rm/usr/ bin/python

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

• sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
• sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6.9
  python3.x.x为自己Python安装的版本，有数字的优先级1 python2.7.1 和 2 python3.6.9
• sudo ln - s / usr / bin / python2.7
• sudo update-alternatives --config python
• 按键选择 1
• 然后 roscore