title: "Reduce" function for Series
body: <p>Is there an analog for <code>reduce</code> for a pandas Series?</p>
  
  <p>For example, the analog for <code>map</code> is <a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.apply.html#pandas.Series.apply" rel="noreferrer">pd.Series.apply</a>, but I can't find any analog for <code>reduce</code>. </p>
  
  <hr>
  
  <p>My application is, I have a pandas Series of lists:</p>
  
  <pre><code>&gt;&gt;&gt; business["categories"].head()
  
  0                      ['Doctors', 'Health &amp; Medical']
  1                                        ['Nightlife']
  2                 ['Active Life', 'Mini Golf', 'Golf']
  3    ['Shopping', 'Home Services', 'Internet Servic...
  4    ['Bars', 'American (New)', 'Nightlife', 'Loung...
  Name: categories, dtype: object
  </code></pre>
  
  <p>I'd like to merge the Series of lists together using <code>reduce</code>, like so:</p>
  
  <pre><code>categories = reduce(lambda l1, l2: l1 + l2, categories)
  </code></pre>
  
  <p>but this takes a horrific time because merging two lists together is <code>O(n)</code> time in Python. I'm hoping that <code>pd.Series</code> has a vectorized way to perform this faster.</p>