title: select column with non-zero values from dataframe
body: <p>I have data like the data below.  I would like to only return the columns from the dataframe that contain at least one non-zero value.  So in the example below it would be column ALF.  Returning non-zero rows doesn’t seem that tricky but selecting the column and records is giving me a little trouble.</p>
  
  <pre><code>print df
  
  Data:
  
  Type             ADR             ALE     ALF               AME  
  Seg0              0.0            0.0     0.0              0.0   
  Seg1              0.0            0.0     0.5              0.0 
  </code></pre>
  
  <p>When I try something like the link below:</p>
  
  <p><a href="https://stackoverflow.com/questions/44453059/pandas-how-to-select-columns-with-non-zero-value-in-a-sparse-table">Pandas: How to select columns with non-zero value in a sparse table</a></p>
  
  <pre><code>m1 = (df['Type'] == 'Seg0')
  m2 = (df[m1] != 0).all()
  
  print (df.loc[m1,m2])
  </code></pre>
  
  <p>I get a key error for 'Type'</p>