title: pandas dataframe remove constant column
body: <p>I have a dataframe that may or may not have columns that are the same value. For example</p>
  
  <pre><code>    row    A    B
      1      9    0
      2      7    0
      3      5    0
      4      2    0
  </code></pre>
  
  <p>I'd like to return just</p>
  
  <pre><code>   row    A  
     1      9    
     2      7    
     3      5    
     4      2
  </code></pre>
  
  <p>Is there a simple way to identify if any of these columns exist and then remove them?</p>