How to set Python back to the original version

Solution 1:

The system's default version is the one pointed to by the symlink /usr/bin/python. In 16.04 this should be python2.7 but you have changed it (as you discovered, this is a bad idea as many programs in Ubuntu use Python and will not work if they find the wrong version), and it is pointing to /usr/bin/python3 which is a symlink pointing to /usr/bin/python3.5

Remove the symlink you changed (you can't just overwrite it in one step):

sudo rm /usr/bin/python

and make a new symlink with the same name to the correct Python version (that it originally pointed to). In your case this is python2.7 so:

sudo ln -s /usr/bin/python2.7 /usr/bin/python

now python --version will give you Python 2.7.12 (or 2.7.[some number] depending on what minor version is installed)