title: how to get the column name resulting True in Pandas boolean reduction
body: <p>I have a df,</p>
  
  <pre><code>     0               1              2          A
  -0.740485792    -0.299824912    0.169113705    1
   1.120120949    -0.62580736     0.013757667    2
  -0.685112999     0.439492717    -0.484524907   3
  </code></pre>
  
  <p>I am trying get the column name which has all the values greater than 0,</p>
  
  <p>I tried <code>(df &gt; 0).all()</code></p>
  
  <pre><code>Out[47]: 
   0    False
   1    False
   2    False
   A     True
   dtype: bool
  </code></pre>
  
  <p>How to get only the column name which are True,</p>
  
  <p>My expected output is "A", thanks in advance.</p>
  
  <p>Question 2 on sort_index()</p>
  
  <pre><code> df2 = pd.DataFrame({"A":[3,2,1]}, index=[2,1,0])
  
   Out[395]:
      A
  2   3
  1   2
  0   1
  
  df2.sort_index(axis=1)
  
      A
  2   3
  1   2
  0   1
  </code></pre>
  
  <p>expected output is,</p>
  
  <pre><code>    A
  0   3
  1   2
  2   1
  </code></pre>