local variable referenced before assignment in Python

Question

What am I doing wrong?

from Tkinter import *

coordY = 400

win = Tk()

def key(event):
    if str(event.char) == 'w':
        coordY = coordY - 5
        print coordY

def callback(event):
    canvas.focus_set()


canvas = Canvas(win, width=800, height=450)
canvas.bind("<Key>", key)
canvas.bind("<Button-1>", callback)
canvas.pack()

photo = PhotoImage(file="image.gif")

canvas.create_image(400, coordY, image=photo)

naveY = 400

win.mainloop()

UnboundLocalError: local variable 'naveY' referenced before assignment

I want that when I press 'w' then the image moves up

Answer 1

Not knowing where your error comes from assuming theres more code, there could be a few different places. Does you error you get come from inside a class or function? if so try to set naveY as a global inside your function

global naveY

if your error traces to outside of a function or class is naveY defined before the line thats causing you the error? if so move your

naveY = 400

line accordingly

Answer 2

Not sure where naveY (never mentioned anywhere in your code!) springs from, but here's a bug you have that could cause that exception (with a different variable name):

def key(event):
    if str(event.char) == 'w':
        coordY = coordY - 5
        print coordY

coordY is never defined (Python knows it's a local variable because you're assigning to it within the function), yet you're trying to subtract 5 from it -- what?!-)

I suspect this function is missing a first statement global coordY to let Python know that it's not a local variable -- and that your reported problem with the mysterious naveY is actually this very problem, and you just did some renaming in the code you're showing us, compared with the code you got that exception from:-)