title: Pandas: Summing all elements in a dataframe?
body: <p>Given a Pandas dataframe <code>df</code>, we can sum the columns like this</p>
  <pre><code>[x for x in df.sum()]
  </code></pre>
  <p>and produce the sum of sums like this.</p>
  <pre><code>sum([x for x in df.sum()])
  </code></pre>
  <p>Can this be done using only dataframe operations, without resorting to Python's sum()?</p>