title: Find the unique values in a column and then sort them
body: <p>I have a pandas dataframe. I want to print the unique values of one of its columns in ascending order. This is how I am doing it:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({'A':[1,1,3,2,6,2,8]})
  a = df['A'].unique()
  print a.sort()
  </code></pre>
  
  <p>The problem is that I am getting a <code>None</code> for the output.</p>