How would I open and close another application with a python script?

Updated Answer

From your comments, it seems that the Python app doesn't actually need to know the outcome of the user's check of the PDF, just that it needs to know when he has finished checking.

AFAIK, the easiest way is to use the scripting functionality of TextEdit to ask it for a list of the open documents every second or so. Then you could wait until the user closes the document you opened for him and it will disappear from the list.

The script could look something like this, called GetTextEditDocumentList:

#!/bin/bash
osascript -e 'tell app "TextEdit" to get documents'

It will print a list of open documents on its stdout which you can read with Python. Or you could pass it a filename as a parameter and it could loop, with say a 1 second delay in the middle, until that file/document is no longer open.

You can just try it in the terminal while you open and close a few documents with TextEdit to see how it works.

Original Answer

You appear to be trying to use TextEdit to get some text (a commentary maybe) about a PDF visible in Preview, when a text entry dialog box might be more appropriate.

Whilst you could install, and force any user to install Tkinter or wxPython, you can actually do that very simply using Applescript which is already built into macOS without needing to install anything.

So, save the following as dialog:

#!/bin/bash
input=$(osascript <<EOF
tell application "System Events" 
   display dialog "Enter something:" default answer "Some default answer" with title "Gimme da thing" buttons {"Okey dokey"}
end tell
EOF
)
echo $input

And make it executable with:

chmod +x dialog

Then it will pop up a dialog box like this:

And the output is:

./dialog 
button returned:Okey dokey, text returned:No, I refuse

Then you can very simply call this from Python, using:

subprocess.Popen(['dialog'],stdout=subprocess.PIPE)

Of course, you could parameterize the title, prompt and default text and pass those to the script if you need to be more flexible than the static title I made up.

As far as I know what you want to achieve isn't possible. This is unless you could find a way to catch a character event inside the external program.

I would rather advise you to create your own GUI (Graphical User Interface) in Python, e.g. via wxPython, and then embed a wx.TextCtrl in it so you will have much more control (e.g. it would be easy to catch many types of KeyEvents).

However, to close the application in your example I would do something like:

retrieve_pid_1 = subprocess.Popen(['ps', '-A'], stdout=subprocess.PIPE)
retrieve_pid_2 = subprocess.Popen(['grep', '-m1', 'TextEdit'], stdin=retrieve_pid_1.stdout, stdout=subprocess.PIPE)
retrieve_pid_3 = subprocess.Popen(['awk', '{print $1}'], stdin=retrieve_pid_2.stdout, stdout=subprocess.PIPE)
process_number = retrieve_pid_3.communicate()[0].decode('ASCII').rstrip()
kill_process = subprocess.Popen(['kill', process_number])