Module subprocess: How to "communicate" more than once?

[Edward C. Jones]

I have a program named "octave" (a Matlab clone). It runs in a terminal, 
types a prompt and waits for the user to type something. If I try

# Run octave.
oct = subprocess.Popen("octave", stdin=subprocess.PIPE)
# Run an octave called "startup".
oct.communicate("startup")
# Change directory inside octave.
oct.communicate("cd /home/path/to/my/dir")
I get:
Traceback (most recent call last):
  File "./popen.py", line 29, in ?
    oct.communicate("cd /home/path/to/my/dir")
  File "/usr/local/lib/python2.4/subprocess.py", line 1044, in communicate
    self.stdin.flush()
ValueError: I/O operation on closed file
How do I set up a subprocess so I can send it a command and get the 
answer, then send it another command and get an answer, etc.?
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