Python - Tkinter - create widget to group widgets

#
# https://www.reddit.com/r/learnpython/comments/4uycav/python_and_tkinter_how_to_refresh_gui_upon_button/
#

import Tkinter as tk
import ttk
import tkMessageBox

# --- classes ---

class Combo1(tk.Frame):

    def __init__(self, parent):

        tk.Frame.__init__(self, parent)

        tk.Label(self,
            text = "Choose source(s)."
            ).grid(row=0)

        self.check_var1 = tk.IntVar() # variables lower_case_with_underscore
        self.check_var2 = tk.IntVar() # variables lower_case_with_underscore
        self.check_var3 = tk.IntVar() # variables lower_case_with_underscore

        tk.Checkbutton(self,
                text="1", variable=self.check_var1,
                onvalue=1, offvalue=0, height=1, width=10
                ).grid(row=1)

        tk.Checkbutton(self,
                text="2", variable=self.check_var2,
                onvalue=1, offvalue=0, height=1, width=10
                ).grid(row=2)

        tk.Checkbutton(self,
                text="3", variable=self.check_var3,
                onvalue=1, offvalue=0, height=1, width=10
                ).grid(row=3)


class Combo2(tk.Frame):

    def __init__(self, parent):

        tk.Frame.__init__(self, parent)

        tk.Label(self,
                text = "Choose Wisely."
                ).grid(row=0)

        self.box_value = tk.StringVar()

        self.box = ttk.Combobox(self, textvariable=self.box_value,
                                state='readonly')

        self.box['values'] = ('D', 'E', 'F')
        self.box.current(0)
        self.box.grid(row=1)


class Application(tk.Tk): # tk.Tk creates main window

    def __init__(self):
        tk.Tk.__init__(self)

        self.title("Thingy")
        self.geometry("500x600")

        self.create_options()
        self.combo1 = Combo1(self) # it creates combo but doesn't show
        self.combo2 = Combo2(self) # it creates combo but doesn't show


    def create_options(self):
        '''create and show options'''

        self.widgets = tk.Frame(self)

        tk.Label(self.widgets,
              text="Choose an option."
              ).grid(row=0, column=0)
        tk.Label(self.widgets,
              text="Select one:"
              ).grid(row=1, column=0)

        self.favorite = tk.IntVar()

        tk.Radiobutton(self.widgets,
                    text="Option1", variable=self.favorite,
                    value=1,
                    command=self.show_combo1 # show_combo1
                    ).grid(row=2, column=0)
        tk.Radiobutton(self.widgets,
                    text="Option2", variable=self.favorite,
                    value=2,
                    command=self.show_combo2 # show_combo2
                    ).grid(row=3, column=0)

        self.widgets.grid(column=0)


    def show_combo1(self):
        '''hide combo2 and show combo1'''

        self.combo2.grid_forget()
        self.combo1.grid(row=0, column=1)


    def show_combo2(self):
        '''hide combo1 and show combo2'''

        self.combo1.grid_forget()
        self.combo2.grid(row=0, column=1)


# --- start ---

Application().mainloop()