#!/usr/bin/env python
# -*- coding: utf-8 -*-

from tkinter import ttk
import tkinter as tk

class Application(ttk.Frame):
    
    def __init__(self, main_window):
        super().__init__(main_window)
        main_window.title("Combobox")
        
        self.combo_text = tk.StringVar()
        self.combo_text.trace("w", self.text_changed)
        
        self.combo = ttk.Combobox(self, textvariable=self.combo_text)
        self.combo.place(x=50, y=50)
        
        main_window.configure(width=300, height=200)
        self.place(width=300, height=200)
    
    def text_changed(self, *args):
        "Esta función es invocada cada vez que el texto cambia."
        print("Text changed.")

main_window = tk.Tk()
app = Application(main_window)
app.mainloop()