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https://www.cnblogs.com/yjlch1016/p/8641910.html
调整Python3的优先级，使得3.6优先级较高
sudo update-alternatives –install /usr/bin/python3 python3 /usr/bin/python3.5 1
sudo update-alternatives –install /usr/bin/python3 python3 /usr/bin/python3.6 2
更改默认值，python默认为Python2，现在修改为Python3
sudo update-alternatives –install /usr/bin/python python /usr/bin/python2 100
sudo update-alternatives –install /usr/bin/python python /usr/bin/python3 150
此时再输入命令python
sudo update-alternatives –remove python /usr/bin/python2.7
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sudo update-alternatives –config python3