linux set python 3 as default

C++
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 2
update-alternatives: error: no alternatives for python3 
1. Check current version:
$ python --version
2. get root permissions:
$ sudo su
3. set python3 as default:
$ update-alternatives --install /usr/bin/python python /usr/bin/python3 1
4. Done. (Check by $ python --version)

Source

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