title: Pandas: sum DataFrame rows for given columns
body: <p>I have the following DataFrame:</p>
  <pre><code>In [1]:
  df = pd.DataFrame({'a': [1, 2, 3],
                     'b': [2, 3, 4],
                     'c': ['dd', 'ee', 'ff'],
                     'd': [5, 9, 1]})
  
  df
  Out [1]:
     a  b   c  d
  0  1  2  dd  5
  1  2  3  ee  9
  2  3  4  ff  1
  </code></pre>
  <p>I would like to add a column <code>'e'</code> which is the sum of columns <code>'a'</code>, <code>'b'</code> and <code>'d'</code>.</p>
  <p>Going across forums, I thought something like this would work:</p>
  <pre><code>df['e'] = df[['a', 'b', 'd']].map(sum)
  </code></pre>
  <p>But it didn't.</p>
  <p>I would like to know the appropriate operation with the list of columns <code>['a', 'b', 'd']</code> and <code>df</code> as inputs.</p>