title: Saving only non-null entry value and column number from pandas df with only one non-null value per row
body: <p>I have a pandas dataframe with many columns, most of them are null, but for each row there always is one and only one column with value a string.</p>
  
  <p>I am creating a new column in the dataframe that selects the only non-null value:</p>
  
  <pre><code>data[label] = data.iloc[:,0]  
  for col in range(1,100) :
      data[label] = data[label].fillna(data.iloc[:,col])
  </code></pre>
  
  <p>This works fine, however, I would also keep track of which one of these columns was the non-null, for each entry, so that the column label has that information as well. How do I know which column was non-empty?</p>
  
  <pre><code>Ex.
  
  col0      col1     col2
            "red"
  "blue"
                    "yellow"
  
  new column label is:
  
  label
  "red"/col1
  "blue"/col0
  "yellow"/col2
  </code></pre>