Switching between python 2.7 and 3.5 in Debian

https://linuxconfig.org/how-to-change-default-python-version-on-debian-9-stretch-linux

update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2

*the higher the number at the end, the higher the priority of the alternative

to confirm out settings:

update-alternatives --config python
update-alternatives --list python
python --version