title: Merge multiple column in one column in python
body: <p>I have a pandas data frame where the total of first column is done in second column and same for the second and third column:</p>
  <pre><code>    Column1  Column2  Column3  
   0     3                         
   1     2                         
   2     1       6                 
   3             7        13  
  </code></pre>
  <p>What I want to do now is merge all the columns (1,2,3) into one column A (ignoring the total value) like this:</p>
  <pre><code>     Column A      
   0      3                         
   1      2                         
   2      1                        
   3      7  
  </code></pre>
  <p>How could I best approach this issue? Here is the code mentioned below.</p>
  <pre><code>import pandas as pd
   
  data = {'Column1':[3, 2, 1, &quot;&quot;],
          'Column2': [&quot;&quot;, &quot;&quot;, &quot;6&quot;, &quot;7&quot;],
          'Column3':[&quot;&quot;, &quot;&quot;, &quot;&quot;, 13]}
  
  abc = pd.DataFrame(data)
  
  abc
  
  abc.to_dict()
  </code></pre>
  <p>My Output:</p>
  <pre><code>{'Column1': {0: 3, 1: 2, 2: 1, 3: ''},
   'Column2': {0: '', 1: '', 2: '6', 3: '7'},
   'Column3': {0: '', 1: '', 2: '', 3: 13}}
  </code></pre>