title: Aggregate Monthly Values
body: <p>I have a python list containing multiple list:</p>
  
  <pre><code> A = [['1/1/1999', '3.0'], 
        ['1/2/1999', '4.5'],
        ['1/3/1999', '6.8'],
        ......
        ......
  
        ['12/31/1999', '8.7']]
  </code></pre>
  
  <p>What I need is to combine all the values corresponding to each month, preferably in the form of a dictionary containing months as keys and their values as values. </p>
  
  <p>Example:</p>
  
  <pre><code>   &gt;&gt;&gt; A['1/99']
     &gt;&gt;&gt; ['3.0', '4.5', '6.8'.....]
  </code></pre>
  
  <p>Or in the form of a list of list, so that:</p>
  
  <p>Example:</p>
  
  <pre><code>  &gt;&gt;&gt; A[0]
    &gt;&gt;&gt; ['3.0', '4.5', '6.8'.....]
  </code></pre>
  
  <p>Thanks.</p>