#! /usr/bin/python

'''Calculator in object oriented style,
__future__ to use true division by '/'
'''

from __future__ import division
from Tkinter import \
     Tk, Frame, Entry, Button, \
     END, TOP, LEFT, RIGHT

class GUI:

    def __init__(self, title='My calc',
                 width_root=160, height_root=170,
                 width_num=1, width_math=2,
                 btn_names='C7894561230.=+-/*'):

        self.root = Tk()
        self.root.title(title)
        self.root.maxsize(width = width_root, height = height_root)
        self.root.minsize(width = width_root, height = height_root)
        
        fra = Frame(self.root)
        fra_ent = Frame(self.root)
        fra_math = Frame(self.root)

        fra_num = Frame(fra)
        fra_num2 = Frame(fra)
        fra_num3 = Frame(fra)
        fra_num4 = Frame(fra)

        self.entry = Entry(fra_ent)
        self.entry.insert(END, '0')
        
        self.entry.pack(side = TOP)

        fra_ent.pack(side = TOP)
        fra.pack(side = LEFT)
        fra_math.pack(side = RIGHT)
        
        fra_num.pack(side = TOP)
        fra_num2.pack(side = TOP)
        fra_num3.pack(side = TOP)
        fra_num4.pack(side = TOP)
        
        strct = {btn_names[0]: (fra_ent, width_math, RIGHT)}
        for i in btn_names[1:4]:
            strct[i] = (fra_num, width_num, LEFT)
        for i in btn_names[4:7]:
            strct[i] = (fra_num2, width_num, LEFT)
        for i in btn_names[7:10]:
            strct[i] = (fra_num3, width_num, LEFT)
        for i in btn_names[10:13]:
            strct[i] = (fra_num4, width_num, LEFT)
        for i in btn_names[13:]:
            strct[i] = (fra_math, width_math, TOP)
            
        for name in btn_names:            
            Button(strct[name][0], text = name,                   
                   width = strct[name][1],
                   command = lambda x = name: self.write(x)
                   ).pack(side = strct[name][2])
            
    def write(self, char):
        self.show(char)

    def show(self, value):
        self.entry.delete(first = 0, last = END)
        self.entry.insert(END, value)

    def run(self):
        self.root.mainloop()
        
        
class Calc(GUI):

    def __init__(self):
        GUI.__init__(self)
        self.numb = ''
        self.memr = ''
        self.result = ''
        self.switch = {'=': lambda: self.calculate(),
                       'C': lambda: self.reset(),
                       '.': lambda: self.dot(),                   
                       '+': lambda: self.operation('+'),
                       '-': lambda: self.operation('-'),
                       '*': lambda: self.operation('*'),
                       '/': lambda: self.operation('/')}

    def write(self, char):        
        if char in self.switch:
            self.switch[char]()
        else:
            self.numb += char
            self.show(self.numb)
            
    def reset(self):
        self.numb = ''
        self.memr = ''
        self.result = ''
        self.show('0')

    def dot(self):
        if not self.numb:
            # Has not achieved the number before '.'            
            if self.result:
                self.reset()
            self.numb = '0.'
            self.show(self.numb)
        elif '.' in self.numb:
            # Repeated, incorrect entry '.'
            pass
        else:
            self.numb += '.'
            self.show(self.numb)

    def operation(self, char):
        '''Arithmetic operations'''
        if self.result:
            # Use the result of the previous calculation
            self.numb = self.result
            self.result = ''
        self.memr += self.numb + char
        self.show(self.numb + char)
        self.numb = ''            

    def calculate(self):
        try:
            result = eval('%s%s' % (self.memr, self.numb))
        except (SyntaxError, ZeroDivisionError):
            self.reset()
            self.show('ERROR')
        else:
            self.result = str(result)
            self.numb = ''
            self.memr = ''
            self.show(self.result)

if __name__ == '__main__':
    Calc().run()

def __init__(self):
    GUI.__init__(self)
    self.numb = ''
    self.memr = ''
    self.result = ''

def __init__(self):
    GUI.__init__(self)
    self.reset()

def reset(self):
    self.numb = ''
    self.memr = ''
    self.result = ''

self.switch = {'=': self.calculate,
                       'C': self.reset,
                       '.':  self.dot
...