title: Tkinter Entry: How to make default entry text be deleted when clicked the first time only?
body: <p>i know a question similar to this already exist, but mine is a little different.
  I found an answer on how to make text be deleted when an entry is clicked, but then the text is gone everytime you click the entry. I would like an entry that has a default text "a", and when the entry is clicked the "a" will disappear, but if you then type some new text in and click the entry again, then the text should not be deleted.</p>
  
  <pre><code>import tkinter as tk
  
  root = tk.Tk()
  e = tk.Entry(root)
  e.insert(0, "a")
  
  def some_callback(event): # note that you must include the event as an arg, even if you don't use it.
      e.delete(0, "end")
      return None
  
  e.bind("&lt;Button-1&gt;", some_callback)
  
  e.pack()
  
  root.mainloop()
  </code></pre>
  
  <p>When you run this and click the entry, the entry gets cleared, but if then fx. type 123 and click the entry, the text will be cleared again. So how do I make it so it only gets cleared the first time i click the entry?
  Hope you understand my question, thanks&lt;3</p>