if python2.5 is not automatically installed when you install package which needs it either your repositories are misconfigured or package is broken or you are not installing package from repository.

At least datable is not found from known good repositories (diablo extras and extras-devel)

Both the applications you mentioned are installed from a deb. For whatever reasion, if you install something from a deb, it will NOT satisfy dependencies using the repositories you have installed. Try installing canola or some other python application that shows up in the application manager.