title: How can I tell the program only a certain type of input its allowed?
body: <p>Im working on a program that asks the user to enter input twelve times. Those inputs must be included in a list in which there are included the first twelve letters of the alphabet. </p>
  
  <pre><code>letters=("A","B","C","D","E","F","G","H","I","J","K","L")
  
  def gen_():
  
      s = []
  
      for i in range(1, 13):
  
          in_ = input("Input the note number " + str(i) + ", please\n", )
  
          if in_ in letters:
  
            s.append(in_)
  
            print(" \n"+str(s)+"\n " )
  
          else:
  
            print("not valid")
  
  gen_()
  </code></pre>
  
  <p>I would like to tell the program that if a certain input is not valid, it should ask the user to try again and enter a valid input on that same instance. I tried setting "i" back to the value in which the input was not valid, by subtracting 1 to i, but it didn´t work.</p>
  
  <p>How can I code this? Thanks</p>