title: Pandas division of two columns with groupby
body: <p>This is obviously simple, but as a pandas newbe I'm getting stuck.</p>
  
  <p>I have a CSV file that contains 3 columns, the State, bene_1_count, and bene_2_count.</p>
  
  <p>I want to calculate the ratio of 'bene_1_count' and 'bene_2_count' in a given state.</p>
  
  <pre><code>df = pd.DataFrame({'state': ['CA', 'WA', 'CO', 'AZ'] * 3,
             'bene_1_count': [np.random.randint(10000, 99999)
                       for _ in range(12)],
             'bene_2_count': [np.random.randint(10000, 99999)
                       for _ in range(12)]})
  </code></pre>
  
  <p>I am trying the following, but it is giving me an error:
         'No objects to concatenate'</p>
  
  <pre><code>df['ratio'] = df.groupby(['state']).agg(df['bene_1_count']/df['bene_2_count'])
  </code></pre>
  
  <p>I am not able to figure out how to "reach up" to the state level of the groupby to take the ratio of columns.</p>
  
  <p>I want the ratio of columns w.r.t a state, like I want my output as follows:</p>
  
  <pre><code>    State       ratio
  
      CA  
      WA  
      CO  
      AZ  
  </code></pre>