pablobarhamal...@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
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