AFAIK, a large chunk of Twisted is already working on Python 3. I don't know how long they have left to get 100% of it, but it's made substantial progress.
From what I gather, one of the things holding mercurial back is support for python 2.4, although the support of %-formatting for bytestrings will move the port of mercurial forward.
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Python 3 is still very much not ready for a variety of complicated things, and some large stacks will likely take years of effort to be ported.
Mercurial is an example, I think twisted is another.
AFAIK, a large chunk of Twisted is already working on Python 3. I don't know how long they have left to get 100% of it, but it's made substantial progress.
From what I gather, one of the things holding mercurial back is support for python 2.4, although the support of %-formatting for bytestrings will move the port of mercurial forward.