> The patch breaks C API + binary compatibility for an essential Python
> type - that's not something you can easily undo.

I don't see how it breaks C API compatibility. No officially documented
function has changed, and the accessor macros still work. Am I missing
something?

As for binary compatibility, yes, it does break it, which isn't an
exceptional situation in the development process. We have changed other
"essential types" too -- for example, recently, the PyLong object got
30-bit digits on some systems. Why you think it is hard to undo, I don't
understand.

As for the future ABI PEP, which has not yet been accepted, it does not
mention PyUnicodeObject as part of the structures which are guaranteed
to remain binary-compatible :
http://www.python.org/dev/peps/pep-0384/#structures