How can I return a variable from a function that is opening a file using filedialog after selecting a tkinter button?

Question

All of the tutorials I have seen deomonstrate the tkinter filedialog.askopenfilename function by only using the information collected within the function that is linked to the tkinter button. I can pass information in the function, but I would like to pass variables (filepath, images, etc.) outside the function and have them update variables in my GUI. I have commented out the location I would like to call the variables in main_gui_setup function below. Any help will be appreciated, as it has been very demoralizing not being able to open a file. If this problem persists, my future as a programmer may be limited to creating tic-tac-toe games or instructional videos for Youtube.

from tkinter import *
import tkinter as tk
from tkinter import ttk
from tkinter import filedialog, messagebox
from PIL import ImageTk, Image # was import PIL.Image, PIL.ImageTk
import cv2


def main():
    root = Tk()
    window1 = Window(root, "X-ray Assist", "Give up")
    return None


# can't pass by reference in python

class Window():

    n = 0
    file_path = ""
    img1_info = ""

    def __init__(self, root, title, message):
        self.root = root
        self.root.title(title)
        #self.root.geometry(geometry)
        self.screen_width = root.winfo_screenwidth()
        self.screen_height = root.winfo_screenheight()
        #self.root.attributes('-topmost', 1)

        # SET APP WINDOW SIZE
        scr_size_main = self.scr_size() # create instance of scr_size
        self.root.geometry("%dx%d+%d+%d" % (self.root_width, self.root_height, self.root_x, self.root_y))

        # CREATE MAIN WINDOW GUI
        create_gui = self.main_gui_setup()

        self.root.mainloop()
        
        pass

    def scr_size(self):
        '''Reads monitor size and adjusts GUI frame sizes'''        

        self.root_width = int(self.screen_width*0.52)
        self.root_height = int(self.screen_height*0.9)
        self.root_x = int(self.screen_width*0.23)
        self.root_y = int(self.screen_height*0.02)        
        self.img_ht_full = int(self.screen_height*0.82)   
        self.tools_nb_width = int(self.screen_width*0.22)
        self.tools_nb_height = int(self.screen_height*0.48)     
        self.hist_nb_width = int(self.screen_width*0.22)
        self.hist_nb_height = int(self.screen_height*0.23)

    def open_image(self):
        
        main_win = ttk.Frame(self.root)
        main_win.grid(column=0, row=0)  
        
        self.file_path = filedialog.askopenfilename(initialdir='/', title='Open File', 
            filetypes=(('tif files', '*.tif'), ('all files', '*.*')))
        self.file_path_label = ttk.Label(main_win, text=self.file_path)
        self.file_path_label.grid(column=0, row=0, columnspan=1, sticky="nw", padx=(5,0), pady=1)    
        self.img1_8bit = cv2.imread(self.file_path, 0) #, cv2.IMREAD_ANYDEPTH | cv2.IMREAD_GRAYSCALE)
        #self.img1_8bit_resize = cv2.resize(self.img1_8bit, (self.img_ht_full, self.img_ht_full)) #, interpolation = cv2.INTER_CUBIC)
        #self.img1_height, self.img1_width = self.img1_8bit.shape # not resized for screen
        #img1_info = text = f"{self.img1_height} {self.img1_8bit.dtype} {self.img1_16bit.dtype}"
        #print(self.img1_width, " x ", self.img1_height, " bitdepth = ", self.img1_8bit.dtype)
        
        #img1_info = ttk.Label
        #print(f"{self.img1_height} {self.img1_width} {self.img1_8bit.dtype}")
        #img1_info.grid(column=3, row=1, columnspan=1, sticky="w", padx=(5,0), pady=1)
        
        #img = io.imread(main_win.filename) #scikit
        
        self.img1_16bit = cv2.imread(self.file_path, cv2.IMREAD_ANYDEPTH | cv2.IMREAD_GRAYSCALE)

        #self.img_canvas = tk.Canvas(self.root, width=self.img_ht_full, height=self.img_ht_full)
        #self.img_canvas.grid(column=1, row=2, columnspan=10, rowspan=10, sticky="nw")

        #self.img_canvas.image = ImageTk.PhotoImage(image=Image.fromarray(self.img1_8bit_resize))
        #self.img_canvas.create_image(0,0, image=self.img_canvas.image, anchor="nw")
        
        # .create_line(x1, y1, x2, y2, fill="color")
        #self.img_canvas.create_line((self.img_ht_full/2), 0, (self.img_ht_full/2), (self.img_ht_full), fill="yellow")
        #self.img_canvas.create_line(0, (self.img_ht_full/2), (self.img_ht_full), (self.img_ht_full/2), fill="yellow")
        
    def main_gui_setup(self):
        main_win = ttk.Frame(self.root)
        main_win.grid(column=0, row=0)
        image_win = ttk.Frame(main_win, borderwidth=25, relief="groove", width=self.img_ht_full, height=self.img_ht_full)                 
        image_win.grid(column=1, row=2, columnspan=10, rowspan=10, sticky="nw")
        toolbar = ttk.Frame(main_win, borderwidth=5) #, width=1100, height=15)
        toolbar.grid(column=0, row=0, columnspan=10, rowspan=1, sticky="nw", padx=20)
        hist_win = ttk.Frame(main_win, borderwidth=25, relief="groove", width=300, height=200)
        panel_info = ttk.Label(main_win, text=f"{self.screen_width} x {self.screen_height}")
        panel_info.grid(column=5, row=1, columnspan=1, sticky="e", pady=1)
        # SCROLL SLIDER AT BOTTOM
        slider = ttk.Scrollbar(main_win, orient="horizontal")
        slider.grid(column=1, row=13, columnspan=7, padx=5, pady=5, sticky="ew")
        #X-RAY AND DETECTOR SETTINGS - will input these from separate class
        kv = ttk.Label(main_win, text="125kV") 
        kv.grid(column=0, row=2, columnspan=1, padx=15, pady=5)
        file_path_label = ttk.Label(main_win, text="No image loaded")
        file_path_label.grid(column=1, row=1, columnspan=1, sticky="nw", padx=(5,0), pady=1)  
        
        # CREATE BUTTONS
        open = ttk.Button(toolbar, text="Open", width=10, command=self.open_image)
        open.grid(column=0, row=0)
        save = ttk.Button(toolbar, text="Save", width=10)
        save.grid(column=1, row=0)
        b1 = ttk.Button(toolbar, text="1", width=10)
        b1.grid(column=2, row=0)
        b2 = ttk.Button(toolbar, text="2", width=10)
        b2.grid(column=3, row=0)

    pass

main()

Answer 1

You aren't thinking of event driven programming correctly. In event driven programming you have callbacks to the functions you defined. Let's look at your code:

def get_path(self):
    ...
    self.path_label = ...
    ...
    
def main_gui_setup(self):
    main_win = ttk.Frame(self.root)
    main_win.pack()
    open = ttk.Button(main_win, text="Open", width=10, command=self.get_path)
    open.pack()

    # Problematic code:
    # main_label = ttk.Label(main_win, self.path_label)
    # main_label.pack()

When main_gui_setup is called it creates a frame and a button inside it. When the button is clicked it calls get_path which sets up the path_label variable. The problem that you were facing is that that as soon as you create your button (without waiting for the button to be pressed), you create the label called main_label.

For a simple fix to your problem try this:

def get_path(self):
    ...
    self.file_path = ...
    self.path_label = ...
    ...

def button_callback(self, main_win):
    # set up
    self.get_path()
    # My guess is that you wanted `self.file_path` here instead of `self.path_label`
    main_label = ttk.Label(main_win, self.file_path)
    main_label.pack()

def main_gui_setup(self):
    main_win = ttk.Frame(self.root)
    main_win.pack()
    # I am using a lambda and passing in `main_win` because you haven't
    # assigned it to `self` using `self.main_win = ...`
    open = ttk.Button(main_win, text="Open", width=10, command=lambda: self.button_callback(main_win))
    open.pack()

Answer 2

I am still confused by what You are trying to accomplish:

from tkinter import Tk, Button, Label, filedialog


class MainWindow(Tk):
    def __init__(self):
        Tk.__init__(self)

        self.open_file_btn = Button(self, text='Open', command=self.open_file)
        self.open_file_btn.pack()
        
        self.file_name = None

    def open_file(self):
        self.file_name = filedialog.askopenfilename()
        Label(self, text=self.file_name).pack()


root = MainWindow()
root.mainloop()

I will explain what will happen here! (You can change the .askopenfilename() attributes obviously).

When the program opens the filedialog and You select a file, that file name will now get assigned to self.file_name and that variable can be used anywhere in the class.

also from what I have seen You should learn more about classes and PEP 8