title: Iterrows a rolling sum
body: <p>I have a <strong>pandas</strong> dataframe </p>
  
  <pre><code>from pandas import DataFrame, Series
  </code></pre>
  
  <p>where each row corresponds to one case, and each column corresponds to one month. I want to perform a rolling sum over each 12 month period. Seems simple enough, but I'm getting stuck with </p>
  
  <pre><code>result = [x for x.rolling_sum(12) in df.iterrows()]
  result = [x for x.rolling_sum(12) in df.T.iteritems()]    
  </code></pre>
  
  <p>SyntaxError: can't assign to function call</p>
  
  <pre><code>a = []
  for x in df.iterrows():
      s = x.rolling_sum(12)
      a.append(s)
  </code></pre>
  
  <p>AttributeError: 'tuple' object has no attribute 'rolling_sum'</p>