title: Python iterator is empty after performing some action on it
body: <p>I was trying to do a challenge on codeeval in python3 and got stuck trying to improve my solution. Every time i tried to iterate (or print, or some other action) two times consecutively over the same iterator, the second loop came up empty. Here is a minimal example that produces this behavior, although I tried several different combinations with lists etc. that gave me the same result:</p>
  
  <pre><code>numbers = ('1','2','3','4','5')
  numbers = map(int, numbers)                                                    
  print(list(numbers))                                                          
  print(list(numbers))
  </code></pre>
  
  <p>results in:</p>
  
  <pre><code>[1, 2, 3, 4, 5]
  []
  </code></pre>
  
  <p>Why does print (in this case) delete the content of numbers? </p>