I need to answer this question: 
How many unique values of quality are in this dataset?

I tried out to answer this question with this function:
df.pd.groupby('quality').unique()

[![enter image description here](https://i.stack.imgur.com/nr4zv.png)](https://i.stack.imgur.com/nr4zv.png)

However, there is a mistake. 
[![enter image description here](https://i.stack.imgur.com/4TZ11.png)](https://i.stack.imgur.com/4TZ11.png)

What should I do?