[eesmith]

I believe that would have been much more expensive to develop, and the Python dev team didn't have the resources for it.

The easiest things can be done by automatic translation, like 2to3.

The remaining parts get increasingly difficult, because Python 2 objects have to know how they are interpreted in Python 3, and vice versa.

A simper step would be to write an AST transformer to instrument Python 2 code and identify the non-Python 3-compatible uses of, for example, keys() and values(). That's almost as hard as writing a Python 3 implementation in Python 2, and I couldn't convince myself that it made financial sense given that it would still miss a lot of corner cases.