For the transition from Python 2 to 3 the Python team released just such a conversion tool, aptly named "2to3." [1] This was better than handy but the transition from 2 to 3 was still lengthy and painful. In practice many of the suggested changes were unnecessary or at least needed human review (e.g. suggesting changing "for n in range(1,11)" to "for n in list(range(1,11))". Thanks, 2to3, but that code is still correct and actually faster with the iterator version of range().) Another problem was that, just like it says on the tin, 2to3 produces a version of code that only runs under Python 3, whereas most library authors during the transition time wanted a version that would run under both 2 and 3 a la the six[2] library.

So while a one-way automated refactoring script sounds like a good idea, and is strictly better than just breaking backwards compatibility and expecting everyone to deal, it wasn't an unqualified success for Python.

[1]: https://docs.python.org/3/library/2to3.html [2]: https://pythonhosted.org/six/