import tkinter as tk

def tk_06():
    """Create a window that will ask the user to enter a number in a text box.
       When they click on a button it will use the code variable.isdigit() to
       check to see if it is a whole number. If it is a whole number, add it to
       a list box, otherwise clear the entry box. Add another button that will
       clear the list."""
    def add_num():
        name = txt_in_box.get()
        if name.isdigit():
            lst_nums.insert('end', name)
            txt_in_box.delete(0, 'end')
            txt_in_box.focus()
        else:
            txt_in_box.delete(0, 'end')
            txt_in_box.focus()

    def reset():
        txt_in_box.delete(0, 'end')
        lst_nums.delete(0, 'end')
        txt_in_box.focus()

    window = tk.Tk()
    window.title("Lista de números TK")
    window.geometry("390x350")

    lbl_info = tk.Label(text="Ingresa un número: ")
    lbl_info.place(x=40, y=25, width=150, height=35)

    txt_in_box = tk.Entry()
    txt_in_box["justify"] = "center"
    txt_in_box.place(x=180, y=25, width=150, height=35)
    txt_in_box.focus()

    btn_add = tk.Button(text="Agregar", command=add_num)
    btn_add.place(x=40, y=75, width=150, height=35)
    btn_reset = tk.Button(text="Vaciar", command=reset)
    btn_reset.place(x=190, y=75, width=150, height=35)

    lst_nums = tk.Listbox()
    lst_nums["justify"] = "center"
    lst_nums.place(x=40, y=120, width=300, height=200)
    
    window.mainloop()