title: How to get the number of unique combinations of two columns that occur in a python pandas dataframe
body: <p>Let's say I have this dataframe in pandas</p>
  
  <pre><code>     a    b
  1    203  487
  2    876  111
  3    203  487
  4    876  487
  </code></pre>
  
  <p>There are more columns that I don't care about not shown</p>
  
  <p>I know <code>len(df.a.unique())</code> will return 2 to indicate there are two unique values of a, as will <code>len(df.b.unique())</code>. I want something similar to this, but returns the number of unique combinations of a AND b that occur. So in this example, I would want it to return 3. </p>
  
  <p>Any guidance on how I can go about doing this are appreciated</p>