tkinter in many files using classes

# screenshot: http://imgur.com/5upiVMQ

### main.py ###

from main_window import MainWindow


mainWindow = MainWindow()
mainWindow.mainloop()



### main_window.py ###

import tkinter
from calc_widget import CalcWidget
from next_window import NextWindow


class MainWindow(tkinter.Tk):

    def __init__(self):
        super().__init__()
        self.create_gui()

    def create_gui(self):
        self.calc_1 = CalcWidget(self, 'Add', command=lambda x, y: x+y)
        self.calc_1.grid(row=0, column=0)

        self.calc_2 = CalcWidget(self, 'Substract', command=lambda x, y: x-y)
        self.calc_2.grid(row=1, column=0)

        self.calc_3 = CalcWidget(self, 'Multiply', command=lambda x, y: x*y)
        self.calc_3.grid(row=2, column=0)

        self.calc_4 = CalcWidget(self, 'Divide', command=lambda x, y: x/y)
        self.calc_4.grid(row=3, column=0)


        self.button_next_window = tkinter.Button(self, text='Show next window', command=self.show_next_window)
        self.button_next_window.grid(row=4, column=0)

        self.button_close = tkinter.Button(self, text='Quit', command=self.destroy)
        self.button_close.grid(row=5, column=0)

    def show_next_window(self):
        next_window = NextWindow(self)



### calc_widget.py ###

import tkinter


class CalcWidget(tkinter.Frame):

    def __init__(self, parent, text, command):
        super().__init__(parent)

        self.button_text = text
        self.command = command

        self.create_gui()

    def create_gui(self):
        self.entry1 = tkinter.Entry(self)
        self.entry1.grid(row=0, column=0)

        self.entry2 = tkinter.Entry(self)
        self.entry2.grid(row=1, column=0)

        self.button_calc = tkinter.Button(self, text=self.button_text, command=self.calc)
        self.button_calc.grid(row=2, column=0)

        self.label_result = tkinter.Label(self, text='No result')
        self.label_result.grid(row=3, column=0)

    def calc(self):

        arg1 = int(self.entry1.get())
        arg2 = int(self.entry2.get())

        result =  self.command(arg1, arg2)

        text_result = 'Result is: {}'.format(result)

        print(text_result)
        self.label_result.configure(text=text_result)



### next_window.py ###

import tkinter


class NextWindow(tkinter.Toplevel):

    def __init__(self, parent):
        super().__init__(parent)
        self.create_gui()

    def create_gui(self):
        self.label_result = tkinter.Label(self, text='   Hello World!   ')
        self.label_result.grid(row=0, column=0)

        self.button_calc = tkinter.Button(self, text='Quit', command=self.destroy)
        self.button_calc.grid(row=1, column=0)