What's wrong with this code? (UnboundLocalError: local variable referenced before assignment)

[pablobarhamalzas at gmail.com]

Hi there! I'm quite new to programming, even newer in python (this is actually the first thing I try on it), and every other topic I've seen on forums about my problem doesn't seem to help. 

So, the following lines are intended to draw a white square (which it does), turn it to blue when you click on it, and back to white when you click on it again (and so on). Here's what I wrote (python 3 syntax):


from tkinter import *

root = Tk()
root.geometry("500x500")

w = Canvas(root, width=500, height=500)
w.pack()

coords = (x1, y1, x2, y2) = (100, 100, 200, 200)

rect = w.create_rectangle(coords, fill="white")    
isWhite = True
        
def change(event):
    if event.x > x1 and event.x < x2 and event.y > y1 and event.y < y2:
        if isWhite:
            w.itemconfig(rect, fill="blue")
            isWhite = False
        else:
            w.itemconfig(rect, fill="white")   
            isWhite = True
                              
w.bind("<Button-1>", change)
      
root.mainloop()  


The problem occurs when clicking on the white square. The following error appears:
"if isWhite:
UnboundLocalError: local variable 'isWhite' referenced before assignment"

However, the isWhite variable is clearly defined at "True" a few lines before.
Also, if I remove the lines that change isWhite to False if it's True and viceversa, the program doesn't throw any error, but obviously doesn't do what I want it to do (it only changes the square color once, as isWhite stays set to True).

What can the problem be? I'm sure it's something really simple, but I don't get it... Thank's!