I was trying to set default python version to python3 in Ubuntu 16.04. By default it is python2 (2.7). I followed below steps :
python3
Ubuntu 16.04
python2
As an added extra, you can add an alias for pip as well (in .bashrc or bash_aliases):
alias pip='pip3'
You many find that a clean install of python3 actually points to python3.x so you may need:
alias pip='pip3.6' alias python='python3.6'