How to receive file when file is opened in python

I am trying to create an app that could open files, like a text editor, in python. But I am running into a problem; I have no way of knowing what file to open. Ex. If I am on my Mac's Finder, and I want to open a file, my app should be an app I could open said file with. My first assumption was that it is passed as an argument when the app is launched. But after looking at the result of sys.argv, I don't see the file. It doesn't matter if I get the file path or the content of the file. I don't know how else to access it. Any help would be appreciated!

This is a base example of my code:

import sys


# This doesn't work!
args = sys.argv
if len(args) > 1:
    file = args[1]
else:
    file = "No file was provided"
# -------------------------------

# Display file

(I am compiling my app using py2app, and using tkinter but am open to alternatives)

For tkinter, the Tk() root has functionality for capturing different events from the OS. The event for opening documents is ::tk::mac::OpenDocument. To create a callback function to handle the event, use createcommand('::tk::mac::OpenDocument', callback).

Here is a basic example:

import tkinter

root = tkinter.Tk('Open File')

file_text = tkinter.Text(root)
file_text.pack()

def callback(*files):
    for f in files:
        # Open File and write it to the screen
        with open(f) as file:
            text = file.read()
            file_text.insert(tkinter.END , text)

root.createcommand('::tk::mac::OpenDocument', callback)

root.mainloop()