python tk change combo menu

 pythonCopyimport tkinter as tk
from tkinter import ttk

def callbackFunc(event):
     print("New Element Selected")
     
app = tk.Tk() 
app.geometry('200x100')

def changeMonth():
    comboExample["values"] = ["July",
                              "August",
                              "September",
                              "October"
                                ]

labelTop = tk.Label(app,
                    text = "Choose your favourite month")
labelTop.grid(column=0, row=0)

comboExample = ttk.Combobox(app, 
                            values=[
                                    "January", 
                                    "February",
                                    "March",
                                    "April"],
                            postcommand=changeMonth)


comboExample.grid(column=0, row=1)

app.mainloop()