Yes, the Python Package Index (PyPI) seems to have become the Python repo of choice. But I seldom install packages directly from PyPI; almost every package that is reasonably mature has been packaged for Ubuntu, so I usually install from Ubuntu repos.
Running more than one version of Python on a machine SHOULD work. I have Python 2.7 and 3.something on my machine and no problems.
But I have heard from people whose experience has not been good. I don't know of anything like perlbrew to make it less painful. I would probably resort to virtual machines with one version per VM.
I run mostly Red Hat-like distros and use the yum tool as their equivalent of apt-get. When I need a python package that isn't in RPM format, there's easy_install.
I used to do my quick-and-dirty stuff in Perl, but CPAN has a bad habit of delivering broken modules. The python eggs have always seemed to run without problems. I credit a lot of this to the fact that unlike Perl, you don't have to compile a lot of C code to get a module going. So mostly I do only things that use basic Perl modules and are regex-related now. Anything else in the Q&D department I usually do in Python.
I don't normally attempt to switch Python versions on a single machine, but you might want to look at the "alternatives" facility. If Ubuntu has support for alternate versions, that's probably what controls it.
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