title: Error 'AttributeError: 'DataFrameGroupBy' object has no attribute' while groupby functionality on dataframe
body: <p>I have a dataframe <code>news_count</code>. Here are its column names, from the output of <code>news_count.columns.values</code>:</p>
  
  <pre><code> [('date', '') ('EBIX UW Equity', 'NEWS_SENTIMENT_DAILY_AVG') ('Date', '')
    ('day', '') ('month', '') ('year', '')]
  </code></pre>
  
  <p>I need to <code>groupby</code> by year and month and sum values of <code>'NEWS_SENTIMENT_DAILY_AVG'</code>. Below is code I tried, but neither work:</p>
  
  <h3>Attempt 1</h3>
  
  <pre><code>news_count.groupby(['year','month']).NEWS_SENTIMENT_DAILY_AVG.values.sum()
  
  'AttributeError: 'DataFrameGroupBy' object has no attribute' 
  </code></pre>
  
  <h3>Attempt 2</h3>
  
  <pre><code>news_count.groupby(['year','month']).iloc[:,1].values.sum()
  
  AttributeError: Cannot access callable attribute 'iloc' of 'DataFrameGroupBy' objects, try using the 'apply' method
  </code></pre>
  
  <p>Input data:</p>
  
  <pre><code>      ticker       date           EBIX UW Equity    month    year
        field             NEWS_SENTIMENT_DAILY_AVG
           0      2007-05-25                   0.3992      5       2007
           1      2007-11-06                   0.3936      11      2007 
           2      2007-11-07                   0.2039      11      2007
           3      2009-01-14                   0.2881       1      2014
  </code></pre>