Tkinter Programming/Simple Calculator

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~cpp
from Tkinter import *

def frame(root, side):
    w = Frame(root)
    w.pack(side = side, expand=YES, fill=BOTH)
    return w

def button(root, side, text, command=None):
    w = Button(root, text=text, command=command)
    w.pack(side=side, expand=YES, fill=BOTH)
    return w

class calculator(Frame):
    def __init__(self):
        Frame.__init__(self)
        self.pack(expand=YES, fill=BOTH)
        self.master.title('simple calculator')
        self.master.iconname('calc1')

        display = StringVar()
        Entry(self, relief=SUNKEN,
              textvariable=display).pack(side=TOP, expand=YES, fill=BOTH)

        for key in ('123', '456', '789', '-0.'):
            keyF = frame(self, TOP)
            for char in key:
                button(keyF, LEFT, char, lambda w=display, s='%s' % char: w.set(w.get() + s))

        opsF = frame(self, TOP)
        for char in "+-*/=":
            if char == '=':
                btn = button(opsF, LEFT, char)
                btn.bind('<ButtonRelease-1>', lambda e, s = self, w = display: s.calc(w), '+')
            else:
                btn = button(opsF, LEFT, char, lambda w = display, c = char : w.set(w.get() + ' ' + c + ' '))

        clearF = frame(self, BOTTOM)
        button(clearF, LEFT, 'Clr', lambda w=display: w.set(''))

    def calc(self, display):
        try:
            display.set(repr(eval(display.get())))
        except ValueError:
            display.set("Error")

if __name__ == '__main__':
    calculator().mainloop();