title: how to define empty variable in python
body: <p>consider this code:</p>
  
  <pre><code>str = "test"
  def v():
      for i in str:
          k = k + i
          print(k)
  </code></pre>
  
  <p>I don't want to assign <em>k</em> anything at all. I used <code>k = None</code> to define empty variable but that doesn't work.</p>