title: Update label text after pressing a button in Tkinter
body: <p>I was wondering how to change the label text after clicking a button. For example: </p>
  
  <pre><code>from Tkinter import *
  import tkMessageBox
  
  def onclick():
      pass
  
  root = Tk()
  
  root.title("Pantai Hospital")
  
  L1 = Label(root, text='Welcome to Pantai Hospital!')
  L1.pack() 
  L2 = Label(root, text='Login')
  L2.pack() 
  
  L3 = Label(root, text = "Username:")
  L3.pack( side = LEFT, padx = 5, pady = 10)
  username = StringVar()
  E1 = Entry(root, textvariable = username, width = 40)
  E1.pack ( side = LEFT)
  
  L4 = Label(root, text = "Password:")
  L4.pack( side = LEFT, padx = 5, pady = 10)
  password = StringVar() 
  E2 = Entry(root, textvariable = password, show = "*", width = 40)    
  E2.pack( side = LEFT)'`
  </code></pre>
  
  <p>I want to change those labels <code>username</code> and <code>password</code> and the entry field into another different label after clicking a button. How do I do that? </p>