title: Different outcome using pandas nunique() and unique()
body: <p>I have a big <code>DF</code> with <code>10 millions</code> rows and I need to find the unique number for each column.</p>
  
  <p>I wrote the function below:
  (need to return a series)</p>
  
  <pre class="lang-py prettyprint-override"><code>def count_unique_values(df):
      return pd.Series(df.nunique())
  </code></pre>
  
  <p>and I get this output:</p>
  
  <pre class="lang-py prettyprint-override"><code>Area          210
  Item          436
  Element         4
  Year           53
  Unit            2
  Value      313640
  dtype: int64
  </code></pre>
  
  <p>expected result should be value 313641.</p>
  
  <p>when I just do </p>
  
  <pre class="lang-py prettyprint-override"><code>df['Value'].unique()
  </code></pre>
  
  <p>I do get that answer. Didn't figure out why I get less with <code>nunique()</code> just there.</p>