Note that Gregory P. Smith has written

    http://code.google.com/p/python-atfork/

I also started a pure python patch but did not get round it posting it.  (It also implements the fork lock idea.)  I'll attach it here.

How do you intend to handle the propagation of exceptions?  I decided that after

    atfork.atfork(prepare1, parent1, child1)
    atfork.atfork(prepare2, parent2, child2)
    ...
    atfork.atfork(prepareN, parentN, childN)

calling "pid = os.fork()" should be equivalent to

    pid = None
    prepareN()
    try:
        ...
            prepare2()
            try:
                prepare1()
                try:
                    pid = posix.fork()
                finally:
                    parent1() if pid != 0 else child1()
            finally:
                parent2() if pid != 0 else child2()
        ...
    finally:
        parentN() if pid != 0 else childN()