Surely the non-transitive part benefits runtime performance as python does not have to traverse objects and structures? I actually first noticed Python mutability last year where it stopped me from getting 100% unit tests passing, was a bit of a laughing point when I was done as 5 minutes after I left the interview room I realised what I'd done and made some new rules for the future. On the cost side of things in terms of processing speed whilst I'm aware CoW (copy on write) semantics exist in some languages how would you feel about a mnemonic or key-word to make copy or copy on write available to python in a more terse way?)