I installed Python 2.7 from source. A dependency of some packages is python. Is there a way I can prevent the install of a lesser version of Python, or let apt know it's already been fulfilled?
You could get the Debian unstable source package for 2.7 and recompile it. That way you'd have a Debian-packaged Python 2.7.
If you want to keep your source installation, use the equivs package to create a fake python2.7 package to fulfil dependencies. The documentation in the package explains all you need to know (start with
You should always avoid building Python yourself, unless you have a very good reason to. You don't want to mess with the Python environment provided by your distro. If you are just tinkering, always do it in a restricted environemnt, e.g. a VM or virtualenv. Also, why are you installing from source? Python 2.7 ought to be available from your distro, even if not by default (e.g. if you are running Debian 6, enable Testing repository to get it).