[Python] Tkinter Checkbutton doesn't change my variable

I'm trying to use a Checkbutton with a function, my_var never changes but it always call my function.

here the code:

my_var = False
def controllo_carta():
global my_var
print str(my_var)

[...]

c = tk.Checkbutton(toolbar, text="press me",onvalue=True,offvalue=False,variable=my_var,command=controllo_carta)
c.select()
c.pack(side=tk.LEFT,padx=2,pady=2)

print 'my var:' + str(my_var)

[...]


where is my mistake?

thanks!

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