title: Does pandas.Series.unique() preserve order?
body: <p>Simple question I haven't been able to find an answer to yet:</p>
  
  <p>Given a pandas Series, I <em>think</em> the order of values given by Series.unique() is that in which they are first encountered in the series, and <em>not</em> any sort sorted order. I.e. </p>
  
  <pre><code>from pandas import Series
  s = Series(['b','b','b','a','a','b'])
  
  s.unique()
  &gt;&gt;&gt; array(['b', 'a'], dtype=object)
  </code></pre>
  
  <p>This is the behavior I want for my application, but can someone tell me if I'm guaranteed to get this order? The documentation is not clear.</p>