I want to uninstall Python 2.6.5 and then install Python 2.5. I also want to uninstall nosetests new version and install the old one.
OS is Ubuntu.
31 Answer
While this would be best asked on askubuntu I have used the answer outlined here before. The main idea is that you can use the Deadsnakes PPA in combination with virtualenv to use the specific Python version that you need. I use it all the time to build specific package versions of Mercurial to deploy to my remote Linux server and it works perfectly!
You should NOT uninstall or downgrade the default Python on an Ubuntu machine since internal programs and services do rely on it. You will get unexpected results if you do.