How to do it...

In the following example, we will associate a StringVar instance to our entry with the textvariable option; this variable traces write operations with the show_message() method as callback:

import tkinter as tk 
 
class App(tk.Tk): 
    def __init__(self): 
        super().__init__() 
        self.var = tk.StringVar() 
        self.var.trace("w", self.show_message) 
        self.entry = tk.Entry(self, textvariable=self.var) 
        self.btn = tk.Button(self, text="Clear", 
                             command=lambda: self.var.set("")) 
        self.label = tk.Label(self) 
        self.entry.pack() 
        self.btn.pack() 
        self.label.pack() 
 
    def show_message(self, *args): 
        value = self.var.get() 
        text = "Hello, {}!".format(value) if value else "" 
        self.label.config(text=text) 
 
if __name__ == "__main__": 
    app = App() 
    app.mainloop() 

When you type something into the Entry widget, the label updates its text with a message composed with the Tk variable value. For instance, if you type the word Phara, the label will show Hello, Phara!. If the entry is empty, the label will not show any text. To show you how to modify the variable's content programmatically, we added a button that clears the entry when you click on it: