Ubuntu 14.04 LTS comes with both Python version 2.7 and 3.4. The default Python version is 2.7 and update-alternatives is not configured to switch Python versions
$ sudo update-alternatives --config python
update-alternatives: error: no alternatives for python
1. So to configure alternatives for Python 2.7 & 3.4 we need to configure update-alternatives as follows
$ sudo 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
$ sudo 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
2. And now you can switch between Python versions
$ sudo update-alternatives --config python
[sudo] password for linuxuser:
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/bin/python3.4 2 auto mode
* 1 /usr/bin/python2.7 1 manual mode
2 /usr/bin/python3.4 2 manual mode
Press enter to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/bin/python3.4 to provide /usr/bin/python (python) in manual mode
