title: Flatten a Series in pandas, i.e. a series whose elements are lists
body: <p>I have a series of the form:</p>
  
  <pre><code>s = Series([['a','a','b'],['b','b','c','d'],[],['a','b','e']])
  </code></pre>
  
  <p>which looks like</p>
  
  <pre><code>0       [a, a, b]
  1    [b, b, c, d]
  2              []
  3       [a, b, e]
  dtype: object
  </code></pre>
  
  <p>I would like to count how many elements I have in total.
  My naive tentatives like</p>
  
  <pre><code>s.values.hist()
  </code></pre>
  
  <p>or</p>
  
  <pre><code>s.values.flatten()
  </code></pre>
  
  <p>didn't work.
  What am I doing wrong?</p>