title: How to get last group in Pandas' groupBy?
body: <p>I wish to get the last group of my group by:</p>
  
  <pre><code>df.groupby(pd.TimeGrouper(freq='M')).groups[-1]:
  </code></pre>
  
  <p>but that gives the error:</p>
  
  <blockquote>
    <p>KeyError: -1</p>
  </blockquote>
  
  <p>Using <code>get_group</code> is useless as I don't know the last group's value (unless there's a specific way to get that value?). Also I might want to get the last 2 groups, etc</p>
  
  <p>How do I do this?</p>