title: Correct way to set value on a slice in pandas
body: <p>I have a pandas dataframe: data. it has columns ["name", 'A', 'B'] </p>
  
  <p>What I want to do (and works) is:</p>
  
  <pre><code>d2 = data[data['name'] == 'fred'] #This gives me multiple rows
  d2['A'] = 0
  </code></pre>
  
  <p>This will set the column A on the fred rows to 0. 
  I've also done:</p>
  
  <pre><code>indexes = d2.index
  data['A'][indexes] = 0
  </code></pre>
  
  <p>However, both give me the same warning: </p>
  
  <pre><code>/Users/brianp/work/cyan/venv/lib/python2.7/site-packages/pandas/core/indexing.py:128: SettingWithCopyWarning: 
  A value is trying to be set on a copy of a slice from a DataFrame
  
  See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  </code></pre>
  
  <p>How does pandas WANT me to do this?</p>