Getting "UnboundLocalError: local variable 'location_y' referenced before assignment" even though it's not

So I’m trying to create a D&D map viewer (so if that I can make the map interactive) but for some reason I keep getting "UnboundLocalError: local variable ‘location_y’ referenced before assignment" and "UnboundLocalError: local variable ‘location_x’ referenced before assignment" and it doesn’t make any sense to me so what do I need to add or change to make this work?

Code:

import PIL.Image
from PIL import ImageTk
from tkinter import *

location_x = 0
location_y = 0
scroll_wheel = 1

root = Tk()
canvas = Canvas(root, width=1920, height=1080)
canvas.pack()

file_name = ("MAP_X" + str(location_x) + "_Y" + str(location_y) + ".png")

def onKeyPress(event):
    if(event.char == "a"):
        location_x -= 1
        if(location_x == -1):
            done_check_x = 0
            while(done_check_x == 0):
                if(PIL.Image.open("MAP_X" + str(location_x) + "_Y" + str(location_y) + ".png")):
                    location_x += 1
                else:
                    location_x -= 1
                    done_check_x = 1

    elif(event.char == "d"):
        location_x += 1
        if not (PIL.Image.open("MAP_X" + str(location_x) + "_Y" + str(location_y) + ".png")):
            location_x = 0

    elif(event.char == "w"):
        location_y -= 1
        if(location_y == -1):
            done_check_y = 0
            while(done_check_y == 0):
                if(PIL.Image.open("MAP_X" + str(location_x) + "_Y" + str(location_y) + ".png")):
                    location_y += 1
                else:
                    location_y -= 1
                    done_check_y = 1
    
    elif(event.char == "s"):
        location_y += 1
        if not (PIL.Image.open("MAP_X" + str(location_x) + "_Y" + str(location_y) + ".png")):
            location_y = 0

root.bind('<KeyPress>', onKeyPress)

while(root):
    file_name = ("MAP_X" + str(location_x) + "_Y" + str(location_y) + ".png")
    img = PIL.Image.open(file_name)
    resized_img = ImageTk.PhotoImage(img.resize((1920 * scroll_wheel, 1080 * scroll_wheel)))
    canvas.create_image(0,0, anchor=NW, image=resized_img)
    
    mainloop()

#with Image.open("MAP_X" + str(start_location_x) + "_Y" + str(start_location_y) + ".png") as img:

Thanks in advance!

>Solution :

Change this:

def onKeyPress(event):
    if(event.char == "a"):

To this:

def onKeyPress(event):
    global location_x, location_y, scroll_wheel

    if(event.char == "a"):