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I have not been able to reproduce the behavior you report using various Pythons on OS X 10.8 or on OS X 10.5. Can you give more specifics on the environment that fails for you?
I certainly don't claim to have any experience with these tty/pty modules or with the underlying system calls and I'm not sure what you are trying to do here but isn't it a bit risky to be calling tty.tcsetattr() from both the parent and child processes simultaneously, since the call is after the fork? tcsetattr is dealing with a job-wide data structure and, under the covers, the tcsetattr() function currently reads and writes various field non-atomically. What happens if you move the tcgetattr and tcsetattr to before the os.fork call or into the parent-only code after the "else"?
http://hg.python.org/cpython/file/default/Modules/termios.c