title: How many unique values of quality are in this dataset?
body: <p>I need to answer this question: 
  How many unique values of quality are in this dataset?</p>
  
  <p>I tried out to answer this question with this function:
  df.pd.groupby('quality').unique()</p>
  
  <p><a href="https://i.stack.imgur.com/nr4zv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nr4zv.png" alt="enter image description here"></a></p>
  
  <p>However, there is a mistake. 
  <a href="https://i.stack.imgur.com/4TZ11.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4TZ11.png" alt="enter image description here"></a></p>
  
  <p>What should I do?</p>