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Date: Mon, 24 Jun 2013 22:10:15 +0200
From: Antoon Pardon <antoon.pardon@rece.vub.ac.be>
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To: python-list@python.org
Subject: Re: What's wrong with this code? (UnboundLocalError: local variable referenced before assignment)
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Op 24-06-13 21:47, pablobarhamalzas@gmail.com schreef:
> 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.

No it is not.

In Python, when you assign to a variable within a function, that
variable will be treated as a local variable. If you have a global
variable with the same name, that global variable will just for
the duration of the function become inaccessible.

The quick solution in this case is to include a global statement.
Something like

def change(event)
     global isWhite
     ...

-- 
Antoon Pardon