title: Drop a column in pandas if all values equal 1?
body: <p>How do I drop columns in pandas where all values in that column are equal to a particular number? For instance, consider this dataframe:</p>
  
  <pre><code>df = pd.DataFrame({'A': [1, 1, 1, 1],
                     'B': [0, 1, 2, 3],
                     'C': [1, 1, 1, 1]})
  print(df)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>   A  B  C
  0  1  0  1
  1  1  1  1
  2  1  2  1
  3  1  3  1
  </code></pre>
  
  <p>How would I drop the <code>1</code> columns so that the output is: </p>
  
  <pre><code>   B
  0  0
  1  1
  2  2
  3  3
  </code></pre>