title: Add a value to the end of a pandas index object
body: <p>I have a pandas index object and I'd like to add a single value to the end of it. The .append() method doesn't seem to work like one would expect, and since I'm trying to add an element, I can't insert at the location of -1 because that puts the value in the second-to-last position. For example</p>
  
  <pre><code>import pandas as pd
  ser = pd.Series([1,2,3,4,5], index=[11,12,13,14,15])
  indx = ser.index
  </code></pre>
  
  <p>Say I want to add the value 20 to the end of the index. This throws an error:</p>
  
  <pre><code>indx.append(20)
  </code></pre>
  
  <p>This returns [11,12,13,14,20,15]:</p>
  
  <pre><code>indx.insert(-1, 20)
  </code></pre>
  
  <p>This works but seems like a work-around:</p>
  
  <pre><code>indx.insert(len(indx), 20)
  </code></pre>
  
  <p>Is there something I'm missing? This is on pandas 0.18.1. Thanks.</p>