ROS修改：ubuntu系统更改默认python版本（重要操作）
ubuntu18.04+ROS melodic
困扰：ROS用到python2，而各种目标检测源码用到python3，而且各种包也要安在python3的环境下，而不是python2.
解决：基于update-alternatives
1.怎么出现的问题
在安装pytorch时发现安装适用于python3.6以上的pytorch版本时出现找不到对应的pytorch版本，而且提示有老的版本，说明在python版本中pip install默认用了python2.*没用python3.*。
rosmelodic@rosmelodic-virtual-machine:~$ pip install torch==1.8.0+cpu torchvision==0.9.0+cpu torchaudio==0.8.0 -f https://download.pytorch.org/whl/torch_stable.html
Collecting torch==1.8.0+cpu
Could not find a version that satisfies the requirement torch==1.8.0+cpu (from versions: 0.1.6.post17, 0.1.6.post20, 0.1.6.post22, 0.1.7.post2, 0.1.8.post1, 0.1.9.post1, 0.1.9.post2, 0.1.10.post1, 0.1.10.post2, 0.1.11.post4, 0.1.11.post5, 0.1.12.post1, 0.1.12.post2, 0.2.0.post1, 0.2.0.post2, 0.2.0.post3, 0.3.0, 0.3.0.post2, 0.3.0.post3, 0.3.0.post4, 0.3.1, 0.4.0, 0.4.1, 1.0.0, 1.0.1, 1.0.1.post2, 1.1.0, 1.2.0, 1.2.0+cpu, 1.2.0+cu92, 1.3.0, 1.3.0+cpu, 1.3.0+cu100, 1.3.0+cu92, 1.3.1, 1.3.1+cpu, 1.3.1+cu100, 1.3.1+cu92, 1.4.0, 1.4.0+cpu, 1.4.0+cu100, 1.4.0+cu92, 1.5.0+cpu, 1.5.0+cu92)
No matching distribution found for torch==1.8.0+cpuubuntu18.04,python版本
rosmelodic@rosmelodic-virtual-machine:~$ python
Python 2.7.17 (default, Jul 1 2022, 15:56:32)
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> rosmelodic@rosmelodic-virtual-machine:~$ python3
Python 3.6.9 (default, Jun 29 2022, 11:45:57)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> ubuntu20.04，python版本
rosnoetic@rosnoetic-virtual-machine:~/桌面$ python
找不到命令“python”，您的意思是：
command 'python3' from deb python3
command 'python' from deb python-is-python3
rosnoetic@rosnoetic-virtual-machine:~/桌面$ python3
Python 3.8.10 (default, Jun 22 2022, 20:18:18)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
ubuntu18.04有两种python版本且默认为2.7.17和3.6.9。
ubuntu20.04有仅支持python3，版本为3.8.10。
由于网上大部分源码已经弃用了python2，故想将ubuntu18.04的默认python版本改为python3，将各种包安装在python3下.
2.修改默认python版本（基于update-alternatives）
ls /usr/bin/python*
rosmelodic@rosmelodic-virtual-machine:~$ ls /usr/bin/python*
/usr/bin/python /usr/bin/python3.6-config
/usr/bin/python2 /usr/bin/python3.6m
/usr/bin/python2.7 /usr/bin/python3.6m-config
/usr/bin/python2.7-config /usr/bin/python3-config
/usr/bin/python2-config /usr/bin/python3m
/usr/bin/python2-qr /usr/bin/python3m-config
/usr/bin/python3 /usr/bin/python-config
/usr/bin/python3.6
python2rosmelodic@rosmelodic-virtual-machine:~$ python2
Python 2.7.17 (default, Jul 1 2022, 15:56:32)
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
KeyboardInterrupt
>>> exit()
python3.6rosmelodic@rosmelodic-virtual-machine:~$ python3.6
Python 3.6.9 (default, Jun 29 2022, 11:45:57)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 2
rosmelodic@rosmelodic-virtual-machine:~$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 1
[sudo] rosmelodic 的密码：
update-alternatives: 使用 /usr/bin/python3.6 来在自动模式中提供 /usr/bin/python (python)
rosmelodic@rosmelodic-virtual-machine:~$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 2
update-alternatives: 使用 /usr/bin/python2 来在自动模式中提供 /usr/bin/python (python)
sudo update-alternatives --config python
rosmelodic@rosmelodic-virtual-machine:~$ sudo update-alternatives --config python
有 2 个候选项可用于替换 python (提供 /usr/bin/python)。
选择 路径 优先级 状态
------------------------------------------------------------
* 0 /usr/bin/python2 2 自动模式
1 /usr/bin/python2 2 手动模式
2 /usr/bin/python3.6 1 手动模式
要维持当前值[*]请按<回车键>，或者键入选择的编号：2
update-alternatives: 使用 /usr/bin/python3.6 来在手动模式中提供 /usr/bin/python (python)
rosmelodic@rosmelodic-virtual-machine:~$ python
Python 3.6.9 (default, Jun 29 2022, 11:45:57)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
修改默认版本成功。
sudo apt install python3-pip
pip install torch==1.8.0+cpu torchvision==0.9.0+cpu torchaudio==0.8.0 -f https://download.pytorch.org/whl/torch_stable.html在安装，成功。
在ROS-melodic中用的是python2，故在安装完各种包时，要把默认的python版本调回来，就是把优先级调一下。不然会报错：
rosmelodic@rosmelodic-virtual-machine:~/catkin_ws/src/yolov5-6.0$ roscore
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'
网上各种删除或者降级python版本，属于杀鸡取卵的方法，如果这么做，刚刚下载的各种pyhon依赖的包因为python版本被破坏而失效，调用这些包的代码也会报错。
灵活的调回来，当需要往python3安装新的包时，再调过去。
sudo update-alternatives --config pythonrosmelodic@rosmelodic-virtual-machine:~/catkin_ws/src/yolov5-6.0$ python
Python 2.7.17 (default, Jul 1 2022, 15:56:32)
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
rosmelodic@rosmelodic-virtual-machine:~/catkin_ws/src/yolov5-6.0$ roscore
... logging to /home/rosmelodic/.ros/log/a70de31c-4607-11ed-89a7-000c2969e545/roslaunch-rosmelodic-virtual-machine-19473.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.
started roslaunch server http://rosmelodic-virtual-machine:46389/
ros_comm version 1.14.13
SUMMARY
========
PARAMETERS
* /rosdistro: melodic
* /rosversion: 1.14.13
NODES
auto-starting new master
process[master]: started with pid [19483]
ROS_MASTER_URI=http://rosmelodic-virtual-machine:11311/
setting /run_id to a70de31c-4607-11ed-89a7-000c2969e545
process[rosout-1]: started with pid [19494]
started core service [/rosout]
roscore没问题。
如果要用到刚刚python3搭建的环境，运行的python文件头要加上：
#!/usr/bin/python3例子：
#!/usr/bin/python3
import rospy
from std_msgs.msg import Int8
import argparse
import os
import sys
from pathlib import Path