title: Assignment to containers in Pandas
body: <p>I want to replace <code>None</code> entries in a specific column in Pandas with an empty list. </p>
  
  <p>Note that some entries in this column may already have an empty list in them, and I don't want to touch those.</p>
  
  <p>I have tried:</p>
  
  <pre><code>indices = np.equal(df[col],None)
  df[col][indices] = []
  </code></pre>
  
  <p>and </p>
  
  <pre><code>indices = np.equal(df[col],None)
  df[col][indices] = list()
  </code></pre>
  
  <p>but both solutions fail with:</p>
  
  <pre><code>ValueError: Length of replacements must equal series length
  </code></pre>
  
  <p>Why? How can I update those specific rows with an empty list?</p>