The copy-on-write semantics are a function of fork, an OS call (which is why it's available in python as os.fork), not a function of python. So if your OS supp

The copy-on-write semantics are a function of fork, an OS call (which is why it's available in python as os.fork), not a function of python. So if your OS supports COW for forked processes, python will have it. Check the man pages for fork(2) and vfork(2) for fork related COW information. There's some interesting stuff in there. As a side note, there's an interesting write-up on the portability of fork between UNIX and win32 in the perlfork man page (forgive me for mentioning perl in a python related thread), and the hoops that needed to be jumped through to emulate fork on win32 and keep it compatible with real fork from the visibility of the perl script itself.