title: Python pandas: select columns with all zero entries in dataframe
body: <p>Given a dataframe how to find out all the columns that only have 0 as the values?</p>
  
  <pre><code>df
     0  1  2  3  4  5  6  7
  0  0  0  0  1  0  0  1  0
  1  1  1  0  0  0  1  1  1
  </code></pre>
  
  <p>Expected output</p>
  
  <pre><code>   2  4
  0  0  0
  1  0  0
  </code></pre>