To change python version system-wide we can use update-alternatives command. Logged in as a root user, first list all available python alternatives:
# update-alternatives --list python
update-alternatives: error: no alternatives for python
The above error message means that no python alternatives has been recognized by update-alternatives command. For this reason we need to update our alternatives table and include both python2.7 and python3.4:
# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode
# update-alternatives --install /usr/bin/python python /usr/bin/python3.4 2
update-alternatives: using /usr/bin/python3.4 to provide /usr/bin/python (python) in auto mode
The --install option take multiple arguments from which it will be able to create a symbolic link. The last argument specified it priority means, if no manual alternative selection is made the alternative with the highest priority number will be set. In our case we have set a priority 2 for /usr/bin/python3.4 and as a result the /usr/bin/python3.4 was set as default python version automatically by update-alternatives command.
# python --version
Python 3.4.2
Next, we can again list all python alternatives:
# update-alternatives --list python
/usr/bin/python2.7
/usr/bin/python3.4
From now on, we can anytime switch between the above listed python alternative versions using below command and entering a selection number:
# update-alternatives --config python