title: How to merge/combine columns in pandas?
body: <p>I have a (example-) dataframe with 4 columns:</p>
  
  <pre><code>data = {'A': ['a', 'b', 'c', 'd', 'e', 'f'],
      'B': [42, 52, np.nan, np.nan, np.nan, np.nan],  
      'C': [np.nan, np.nan, 31, 2, np.nan, np.nan],
      'D': [np.nan, np.nan, np.nan, np.nan, 62, 70]}
  df = pd.DataFrame(data, columns = ['A', 'B', 'C', 'D'])
  
      A   B       C       D
  0   a   42.0    NaN     NaN
  1   b   52.0    NaN     NaN
  2   c   NaN     31.0    NaN
  3   d   NaN     2.0     NaN
  4   e   NaN     NaN     62.0
  5   f   NaN     NaN     70.0
  </code></pre>
  
  <p>I would now like to merge/combine columns B, C, and D to a new column E like in this example:</p>
  
  <pre><code>data2 = {'A': ['a', 'b', 'c', 'd', 'e', 'f'],
      'E': [42, 52, 31, 2, 62, 70]}
  df2 = pd.DataFrame(data2, columns = ['A', 'E'])
  
      A   E
  0   a   42
  1   b   52
  2   c   31
  3   d   2
  4   e   62
  5   f   70
  </code></pre>
  
  <p>I found a quite similar question <a href="https://stackoverflow.com/questions/39276249/merge-two-dataframe-columns-into-1-in-pandas">here</a> but this adds the merged colums B, C, and D at the end of column A:</p>
  
  <pre><code>0      a
  1      b
  2      c
  3      d
  4      e
  5      f
  6     42
  7     52
  8     31
  9      2
  10    62
  11    70
  dtype: object
  </code></pre>
  
  <p>Thanks for help.</p>