title: updating columns based on conditions in pandas 0.16
body: <p>I am trying to update a column based on condition of another column</p>
  
  <pre><code>df=pd.DataFrame(np.random.randn(6,4),columns=list('abcd'))
  df[df.b&gt;0].d=1
  </code></pre>
  
  <p>why doesnt this work? without the condition it works.</p>