Keeping a pipe to a process open

I have an app that reads in stuff from stdin and returns, after a newline, results to stdout

A simple (stupid) example:

$ app
Expand[(x+1)^2]<CR>
x^2 + 2*x + 1
100 - 4<CR>
96

Opening and closing the app requires a lot of initialization and clean-up (its an interface to a Computer Algebra System), so I want to keep this to a minimum.

I want to open a pipe in Python to this process, write strings to its stdin and read out the results from stdout. Popen.communicate() doesn't work for this, as it closes the file handle, requiring to reopen the pipe.

I've tried something along the lines of this related question: Communicate multiple times with a process without breaking the pipe? but I'm not sure how to wait for the output. It is also difficult to know a priori how long it will take the app to finish to process for the input at hand, so I don't want to make any assumptions. I guess most of my confusion comes from this question: Non-blocking read on a subprocess.PIPE in python where it is stated that mixing high and low level functions is not a good idea.

EDIT: Sorry that I didn't give any code before, got interrupted. This is what I've tried so far and it seems to work, I'm just worried that something goes wrong unnoticed:

from subprocess import Popen, PIPE
pipe = Popen(["MathPi开发者_开发百科pe"], stdin=PIPE, stdout=PIPE)         
expressions = ["Expand[(x+1)^2]", "Integrate[Sin[x], {x,0,2*Pi}]"] # ...                                                                                        
for expr in expressions:
    pipe.stdin.write(expr)                                                        
    while True:                                                                         
        line = pipe.stdout.readline()                                                    
        if line != '':     
            print line  
        # output of MathPipe is always terminated by ';'                                                                                                                         
        if ";" in line:                                                                  
            break                                                                       

Potential problems with this?

Using subprocess, you can't do this reliably. You might want to look at using the pexpect library. That won't work on Windows - if you're on Windows, try winpexpect.

Also, if you're trying to do mathematical stuff in Python, check out SAGE. They do a lot of work on interfacing with other open-source maths software, so there's a chance they've already done what you're trying to.

Perhaps you could pass stdin=subprocess.PIPE as an argument to subprocess.Popen. This will make the process' stdin available as a general file-like object:

import sys, subprocess

proc = subprocess.Popen(["mathematica <args>"], stdin=subprocess.PIPE, 
                        stdout=sys.stdout, shell=True)
proc.stdin.write("Expand[ (x-1)^2 ]") # Write whatever to the process
proc.stdin.flush()                    # Ensure nothing is left in the buffer
proc.terminate()                      # Kill the process

This directs the subprocess' output directly to your python process' stdout. If you need to read the output and do some editing first, that is possible as well. Check out http://docs.python.org/library/subprocess.html#popen-objects.