from Gui import *
    
def clear():
    display.delete(0, END)

def evaluate():
    expression = display.get()
    clear()
    try:
        value = str(eval(expression))
    except SyntaxError:
        value = 'ERROR'
    display.insert(0, value)

g = Gui()
g.title('calculator')
display = g.en(fill=NONE, justify='right')
g.gr(4, cweights=[1]*4, rweights=[1]*4)
g.bu(text='C', command=clear)
for c in '0/*789-456+123.':
    g.bu(text=c, command=Callable(display.insert, INSERT, c))
g.endgr()
g.bu(text='=', command=evaluate)
g.mainloop()