title: Filter columns based on a value (Pandas): TypeError: Could not compare ['a'] with block values
body: <p>I'm trying filter a DataFrame columns based on a value.</p>
  
  <pre><code>In[41]: df = pd.DataFrame({'A':['a',2,3,4,5], 'B':[6,7,8,9,10]})
  In[42]: df
  Out[42]: 
     A   B
  0  a   6
  1  2   7
  2  3   8
  3  4   9
  4  5  10
  </code></pre>
  
  <p>Filtering columns:</p>
  
  <pre><code>In[43]: df.loc[:, (df != 6).iloc[0]]
  Out[43]: 
     A
  0  a
  1  2
  2  3
  3  4
  4  5
  </code></pre>
  
  <p>It works! But, When I used strings,</p>
  
  <pre><code>In[44]: df.loc[:, (df != 'a').iloc[0]]
  </code></pre>
  
  <p>I'm getting this error: <code>TypeError: Could not compare ['a'] with block values</code></p>