title: how to implement empty string in python
body: <p>I need to be able to prompt the user to enter an empty string so it can check if the answer is correct. but every time I do that I can error saying invalid literal for int() 
  so I need to change my user_input so it can accept int() and strings(). how do I make that possible ? </p>
  
  <pre><code># program greeting 
  print("The purpose of this exercise is to enter a number of coin values") 
  
  print("that add up to a displayed target value.\n") 
  
  print("Enter coins values as 1-penny, 5-nickel, 10-dime,and 25-quarter.") 
  
  print("Hit return after the last entered coin value.")
  
  print("--------------------") 
  
  #print("Enter coins that add up to 81 cents, one per line.")
  
  import sgenrand
  
      #prompt the user to start entering coin values that add up to 81  
      while True: 
          total = 0 
          final_coin= sgenrand.randint(1,99)
  
          print ("Enter coins that add up to", final_coin, "cents, on per line") 
  
          user_input = int(input("Enter first coin: "))
  
          if user_input != 1 and user_input!=5 and user_input!=10 and user_input!=25:
          print("invalid input")
  
          else:
              total = total + user_input
  
  
          while total &lt;= final_coin:
              user_input = int(input("Enter next coin:"))
  
              if user_input != 1 and user_input!=5 and user_input!=10 and user_input!=25:
                  print("invalid input")                
  
              else:
                  total = total + user_input
  
  
          if total &gt; final_coin : 
              print("Sorry - total amount exceeds", (final_coin)) 
  
          elif total &lt; final_coin:
              print("Sorry - you only entered",(total))
  
          else: 
              print("correct")    
  
          goagain= input("Try again (y/n)?:") 
  
          if goagain == "y":
              continue
          elif goagain == "n":
              print("Thanks for playing ... goodbye!" )
              break
  </code></pre>