title: Select a specific group of a grouped dataframe with pandas
body: <p>I have the following dataframe:</p>
  
  <pre><code>df.index = df['Date']
  df.groupby([df.index.month, df['Category'])['Amount'].sum()
  
  Date   Category     Amount     
  1      A            -125.35
         B            -40.00
                      ...
  12     A            505.15
         B            -209.00
  </code></pre>
  
  <p>I would like to report the sum of the Amount for every Category B like:</p>
  
  <pre><code>Date   Category     Amount     
  1      B            -40.00
                      ...
  12     B            -209.00
  </code></pre>
  
  <p>I tried the <code>df.get_group</code> method but this method needs tuple that contains the Date and Category key. Is there a way to filter out only the Categories with B?</p>