import tkinter

class CalculateMPG_GUI:
    def __init__(self):
        self.main_window = tkinter.Tk()

        ## Frames to group widgets
        self.frame1 = tkinter.Frame()
        self.frame2 = tkinter.Frame()
        self.frame3 = tkinter.Frame()
        self.frame4 = tkinter.Frame()

        ## WIDGETS
        self.gallonsPrompt_label = tkinter.Label(self.frame1,
                                                 text='Enter the number '
                                                      'of gallons:')
        self.gallons_entry = tkinter.Entry(self.frame1,
                                           width=4)

        self.milesPrompt_label = tkinter.Label(self.frame2,
                                               text='Enter the number of '
                                                  'miles driven on full tank:')
        self.miles_entry = tkinter.Entry(self.frame2,
                                         width=4)

        ## Pack Widgets
        self.gallonsPrompt_label.pack(side='left')
        self.gallons_entry.pack(side='left')
        self.milesPrompt_label.pack(side='left')
        self.miles_entry.pack(side='left')

        self.descr_label = tkinter.Label(self.frame3,
                                         text='MPG:')

        self.value = tkinter.StringVar()
        self.mpg_label = tkinter.Label(self.frame3,
                                       textvariable=self.value)

        # Pack second & third frame's widgets.
        self.descr_label.pack(side='left')
        self.mpg_label.pack(side='right')

        ## Create the button widgets for the bottom frame.
        self.calc_button = tkinter.Button(self.frame4,
                                          text='Calculate MPG',
                                          command=self.calculate)

        ## Pack the buttons.
        self.calc_button.pack(side='left')

        # Pack the frames.
        self.frame1.pack()
        self.frame2.pack()
        self.frame3.pack()
        self.frame4.pack()

        # Enter the tkinter main loop.
        tkinter.mainloop()

    ## calculate MPG
    def calculate(self):
        gallons = float(self.gallons_entry.get())
        miles = float(self.miles_entry.get())
        MPG = miles / gallons

        self.value.set(MPG)

calc_MPG = CalculateMPG_GUI()