The Google Appengine SDK requires at least version 2.5 of python, or else it spits the dummy. To compound matters, Google has announced that version 1.4.3 of the SDK is the last to support python 2.5, and all newer versions will require python 2.6. [Update: and now you need python 2.7 for multi-threaded apps.]
So how do you install later versions of python without impacting the system default version - which is required for all sorts of vital system services?