title: Pandas.unique() Returning an Arrary with None type Value
body: <p>I´ve created a new column in a Dataframe that contains the categorical feature 'QD' which describes in which "decile" (the 10%, 20, 30% lower values) the value of another feature of the DataFrame is positioned. You can see the DF head below:</p>
  
  <pre><code>    EPS CPI POC Vendeu          Delta       QD
  1   20692   1   19185.30336 0   -1506.69664 QD07
  8   20933   1   20433.27115 0   -499.72885  QD08
  10  20393   1   20808.04948 0   415.04948   QD10
  18  20503   1   19153.45978 0   -1349.54022 QD07
  19  20587   1   20175.31906 1   -411.68094  QD09
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/r9gU8.png" rel="nofollow noreferrer">Data Frame Head</a></p>
  
  <p>The 'QD' column was created through the function below:</p>
  
  <pre><code>minimo = DF['EPS'].min()
  passo = (DF['EPS'].max() - DF['EPS'].min())/10
  
  def get_q(value):
      for i in range(1,11):        
          if value &lt; (minimo + (i*passo)):
              return str('Q' + str(i).zfill(2))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/tyuF0.png" rel="nofollow noreferrer">Function applied on 'Delta'</a></p>
  
  <p>Analyzing this column, I noticed something strange:</p>
  
  <pre><code>AUX2['QD'].unique()
  </code></pre>
  
  <p>out:</p>
  
  <pre><code>array(['QD07', 'QD08', 'QD10', 'QD09', 'QD06', 'QD05', 'QD04', 'QD03',
         'QD02', 'QD01', None], dtype=object)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/lHMl9.png" rel="nofollow noreferrer">'QD' unique values</a></p>
  
  <p>de .unique() method returns an array with an none value on it. At first I thought there was something wrong with the function, but when I tried to grab the position of the none value, look:</p>
  
  <pre><code>AUX2['QD'].value_counts()
  </code></pre>
  
  <p>out:</p>
  
  <pre><code>QD05    852
  QD04    848
  QD06    685
  QD03    578
  QD07    540
  QD08    377
  QD02    318
  QD09    209
  QD10     68
  QD01     61
  Name: QD, dtype: int64
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/R0Pzz.png" rel="nofollow noreferrer">.value_counts()</a></p>
  
  <pre><code>len(AUX2[AUX2['QD'] == None]['QD'])
  </code></pre>
  
  <p>out:</p>
  
  <pre><code>0
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Mm6z0.png" rel="nofollow noreferrer">len()</a></p>
  
  <p>What am I missing here?</p>