using subpocess for both reading and writing simultaneously

[volcano63]

I think the problem is that 'cpt.py' is not an executable, so Popen fails in the execution.
As you have also redirected the stderr to a pipe you cannot see the error message that shall looks like "cpt.py: Not an executable"

stderr definition is for the process itself - not for subprocess.Popen. The script did not start, among other reasons (let's assume that it has execution flag set and shebang line added, as per your advice):

1. Path to the app was not set - like ./cpt.py
   
2. Command string as parameter may only be provided when it is coupled with shell=True; with default value False list is required (as you have shown)