I think the difference here is that there is not, in fact, unrestricted side-effects. Because the STM transaction scope is decided by the interpreter, not by th

I think the difference here is that there is not, in fact, unrestricted side-effects. Because the STM transaction scope is decided by the interpreter, not by the programmer, the interpreter can ensure the current STM transaction is committed immediately before any operation with side-effects, and a new transaction started immediately after. This loses some of the power of STM, certainly, but PyPy's only aiming to maintain internal consistency of interpreter data structures at this point, so this is good enough for them.