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

[Peter Otten]

pablobarhamalzas at gmail.com wrote:

> 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!

Python statically determines the scope of a variable -- if you rebind a name 
it assumes that the variable is local:

>>> def f():
...     print is_white
...     is_white = 42
... 
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
UnboundLocalError: local variable 'is_white' referenced before assignment

The fix is to tell Python that you want to access the global variable:

>>> def f():
...     global is_white
...     print is_white
...     is_white = 42
... 
>>> is_white = "whatever"
>>> f()
whatever
>>> f()
42