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IMHO this is "don't do that then" territory. You're poking around in the inside of file objects, you have to be careful if you do that.

BTW. What are you trying to accomplish? If you set sys.stdout to child_stdin (e.g. "import sys; sys.stdout = child_stdin"), print will write to the 
pipe. If you really want to be sure that the C-level variable stdout writes to the pipe: os.dup2(child_stdout.fileno(), 1). You can then close 
child_stdout, but still have to do the 'os.dup(1)' part if you want to restore the real stdout later on.