using subpocess for both reading and writing simultaneously

[killerrex]

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"

In linux/mac the trick is as easy as adding the next header to cpt.py:

#!/usr/bin/env python3
# The previous line must be the 1st line, no spaces before the #.

And set the executable flag from the command line:

Output:
$> chmod +x cpt.py

For windows, refer to the official faq.

Other option is to run the script calling the python executable with something like:

cmd = "cpt.py"
with subprocess.Popen(['python', cmd], stdout=subprocess.PIPE,stdin=subprocess.PIPE,stderr=subprocess.PIPE) as p:
    txt = p.stdout.readline()  # read the print i (line 2)
    p.stdin.writelines(txt)    # write it (line3)
    a = p.stdout.readline()    # read the print a (line 3)
    b = p.stdout.readline(ans) # read  (line 2) second time
    p.stdin.writelines(b)      # write it(line 3) second time
    c = p.stdout.readline()    # read (ligne 3) second time