Nick, I started a new branch to experiment with immutable Signatures. So far, almost everything works (except a couple unit-tests, that are modifying now immutable Parameters & Signatures) https://bitbucket.org/1st1/cpython/changesets/tip/branch(%22pep362-2%22) I hope tomorrow we get some feedback on this, and if it's positive - I'll finish off the implementation and update the PEP. I hope we still can make it to 3.3. - Yury