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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: Start the external process Iterate for some number of steps Tell the external process to complete the next processing step by writing a new-line character to it's stdin Wait for the external process to signal it has completed the step by writing a new-line character to it's stdout 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?

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