There is no such thing as "Python 2" or "Python 3" modules. There are just modules that will go belly up because they are incompatible with one or another.
I presume you mean that you can’t access your old Python 2 modules from a new Python 3 installation. That’s just because Python installations usually don’t share their modules (i.e. site-packages). You can try to install them using Python 3 (usually just python3 setup.py install) and see if they work or not.
There's a deeper question here: why doesn't the interpreter contain a compatibility layer for the old code? There's no really good reason for the old modules to go belly up in the new interpreter. At least no other than the man months needed to implement that layer. But then, that probably would still be orders of magnitude less than migrating all the libraries that keep people from migrating...
Even PyPy project doesn't have a goal of running py2-compatible and py3-compatible modules inside one interpreter:
> At the end of the project, it will be possible to decide at translation time whether to build an interpreter which supports Python 2.7 or Python 3.2 and both versions will be nightly tested and available from nightly builds.
That’s essentially what __future__ imports are for – things that go further would be simply too much work and Python 3 was about shedding weight, not shipping essentially two interpreters. (while they are very similar from outside, the innards couldn’t be more different)
Cool. So this way porting things to Py3k got easier. That perfectly explains why all the critical libs have been ported fast and no one remembers about Python 2 anymore, doesn't it?
Shedding weight is sometimes a step in a good direction. But we need to draw a line at some point. As the history of Python 3 adoption shows, that point maybe was not optimal.
Edit: forgot an angle:
Also, __future__ is fixing the incompatibility of what people are using with something that they can't use. I'm not sure if it's the right problem to solve.
As it has been pointed out for several times: the adoption of Python 3 is doing just fine. It has never been expected that everybody will be using Python 3 today.
But look at all those current efforts at Canonical, Django, Twisted…it’s not like nothing is happening and we expect a knot to burst. Far from it! Porting started slowly and has gained a momentum by now that has surprised myself. It’s not like we changed the language completely like perl6 did.
