title: Filter out the column having same value throughout all the rows
body: <p>This is my dataset:</p>
  <pre><code>df = {'brand_no':['BH 1', 'BH 2', 'BH 5', 'BH 7', 'BH 6'],
         '1240000601_min':[5.87,5.87,5.87,5.87,np.nan],
         '1240000601_max':[8.87,7.47,10.1,1.9,10.8],
        '1240000603_min':[5.87,np.nan,6.5,2.0,7.8],
         '1240000603_max':[8.57,7.47,10.2,1.0,10.2],
        '1240000604_min':[5.87,5.67,6.9,1.0,7.8],
         '1240000604_max':[8.87,8.87,8.87,np.nan,8.87],
        '1240000605_min':[15.87,15.67,16.9,1.0,17.8],
         '1240000605_max':[18.11,17.47,20.1,1.9,22.6],
        '1240000606_min':[8.12,8.12,np.nan,8.12,np.nan],
         '1240000606_max;':[np.nan,7.47,10.1,1.9,np.nan]}
  # Create DataFrame
  df = pd.DataFrame(df)
  # Print the output.
  df
  </code></pre>
  <p>As you can see the column stays the same except nan.(Because the data is sparse it has nan as well), so I want drop these column which has value same across all the rows. (in this case column 1240000601_min, 1240000604_max and 1240000606_min)</p>
  <p><a href="https://i.stack.imgur.com/jKNtV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jKNtV.png" alt="enter image description here" /></a></p>
  <p>Desired output:
  <a href="https://i.stack.imgur.com/vWCmS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vWCmS.png" alt="enter image description here" /></a></p>
  <p>As we can see here, all the column with same value across all rows are dropped. Pls help get this.</p>