title: Must all Python instance variables be declared in def __init__?
body: <p>Or can they be declared otherwise?</p>
  
  <p>The code below does not work:</p>
  
  <pre><code>class BinaryNode():
      self.parent = None
      self.left_child = None
  </code></pre>
  
  <p>Do they need to be declared in <code>__init__</code>?</p>