title: How to extend/concatenate two iterators in Python
body: <p>I want concatenate two iterators in an efficient way.</p>
  
  <p>Suppose we have two iterators (in Python3)</p>
  
  <pre><code>l1 = range(10)      # iterator over 0, 1, ..., 9
  l2 = range(10, 20)  # iterator over 10, 11, ..., 19
  </code></pre>
  
  <p>If we convert them to lists, it is easy to concatenate like</p>
  
  <pre><code>y = list(l1) + list(l2)  # 0, 1, ,..., 19
  </code></pre>
  
  <p>However, this can be not efficient.</p>
  
  <p>I would like to do something like</p>
  
  <pre><code>y_iter = l1 + l2  # this does not work
  </code></pre>
  
  <p>What is the good way to do this in Python3? </p>