By the way, the existing code in subprocess.Popen (at least on 2.6.7) reads the pipe incorrectly: It doesn't loop to read all the data until EOF -- it only loops over EINTR until it gets a single successful os.read() call.  However, since this is a pipe read (not a real file read), the system doesn't guarantee that the blocking read will read everything up to requested read size or EOF, whichever comes first.  So, the single os.read call could return a partial read, and the subsequent un-pickling of the exception would fail.