title: How to group and filter in python pandaas
body: <p>I have a dataframe:</p>
  <pre><code>df_select = { 'ISIN': ['x123', 'x345' , 'x745' , 'x743', 'x984'], 
                'ISSUER': ['AH'  ,'BZ' , 'NP', 'BZ', 'AH'], 
                'Opt': [5,4,3,2,1],
               'Rating': [5,4,3,4,6],
                'control': [1,1,1,1,1]}
  df= pd.DataFrame(df_select)
  </code></pre>
  <p>I want to filter those records where the sum of rating is larger than 6</p>
  <pre><code>x = df.groupby('ISSUER')['Rating'].sum()
  x = x[x&gt;6]
  </code></pre>
  <p>How do I make the final selection on the pandas (and in general, is this the best way to do this?)</p>
  <pre><code>df[df['ISSUER'] isin (x.index)] 
  </code></pre>
  <p>is not working</p>