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I have a flag in my Python script which specifies whether I setup and use an external process or not. This process is a command called my_command and it takes data from standard input. If I was to run this on the command-line, it would be something like: $ my_command < data > result I want to use a Python script to generate lines of data by modifying standard input and feeding it to my_command . I'm doing something like this: import getopt, sys, os, stat, subprocess # for argument's sake, let's say this is set to True for now # in real life, I use getopt.getopt() to decide whether this is True or False useProcess = True if useProcess: process = subprocess.Popen(['my_command'], stdin=subprocess.PIPE, stdout=subprocess.PIPE) for line in sys.stdin: # parse line from standard input and modify it # we store the result in a variable called modified_line modified_line = line + "foo" # if we want to feed modified_line to my_command, do the following: if useProcess: process.stdin.write(modified_line) # otherwise, we just print the modified line else: print modified_line However, my_command behaves as if it does not receive any data and quits with an error state. What am I doing wrong? EDIT Let's say my Python script is called my_Python_script . Let's say I would normally pass my_command a file called data over standard input: $ my_command < data > result But now I'm passing it to my_Python_script instead: $ my_Python_script < data > some_other_result I want my_Python_script to conditionally set up a subprocess that runs my_command on the contents of data (which are modified by my_Pyt

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