How to send a new command to a subprocess

Earlier I asked a question about how to set up the tkinter gui to get strings from a subprocess without the whole program hanging. It is now functional.

Now I can't figure out how to send newlines to the subprocess. I tried using process.communicate, but I may have used it incorrectly. I also tried This question but self.process.stdin.write('stop\n'.encode())

doesn't seem to work. How to send new commands to python child subprocess?

Relevant code:

self.process = subprocess.Popen([ "python", "-u", dir + "start.py" ], 
    stdout=subprocess.PIPE, 
    stdin=subprocess.PIPE, 
    stderr=subprocess.PIPE, 
    cwd=dir)