title: Checking value of Tkinter Entry widget
body: <p>I have a submit button here: </p>
  
  <pre><code>submit_btn = Button(top, text="Submit", width=10, command=callback)
  submit_btn.grid(row=3, column=1)
  </code></pre>
  
  <p>and as soon as you click it, it should check the entry field above if the input == "Vincent".</p>
  
  <p>This is the entry field:</p>
  
  <pre class="lang-py prettyprint-override"><code>top = Tk()
  user_label = Label(top, text="User Name")
  user_label.grid(row=0, column=0)
  username = Entry(top, bd = 5)
  username.grid(row=0, column=1)
  </code></pre>
  
  <p>Is there a way to check if the value of the 'username' Entry object is "Vincent"?</p>