Starting and Controlling an External Process via STDIN/STDOUT with Python

I need to launch an external process that is to be controlled via messages sent back and forth via stdin and stdout. Using subprocess.Popen I am able to start the process but am unable to control the execution via stdin as I need to.

The flow of what I'm trying to complete is to:

1. Start the external process
2. Iterate for some number of steps
   1. Tell the external process to complete the next processing step by writing a new-line character to it's stdin
   2. Wait for the external process to signal it has completed the step by writing a new-line character to it's stdout
3. Close the external process's stdin to indicate to the external process that execution has completed.

I have come up with the following so far:

process = subprocess.Popen([PathToProcess], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
for i in xrange(StepsToComplete):
    print "Forcing step # %s" % i
    process.communicate(input='\n')

When I run the above code the '\n' is not communicated to the external process, and I never get beyond step #0. The code blocks at process.communicate() and does not proceed any further. I am using the communicate() method incorrectly?

Also how would I implement the "wait until the external process writes a new line" piece of functionality?