title: pandas: error when copy a column from another data frame
body: <p>I am trying to append the prediction column from <code>df_B</code> to <code>df_A</code>.</p>
  
  <pre><code>df_A['prediction'] = df_B['prediction']
  </code></pre>
  
  <p>But the column wasn't properly copied and I got the following error:</p>
  
  <pre><code>/Users/edamame/workspace/git/tensorplay/venv/lib/python3.7/site-packages/ipykernel_launcher.py:1: SettingWithCopyWarning: 
  A value is trying to be set on a copy of a slice from a DataFrame.
  Try using .loc[row_indexer,col_indexer] = value instead
  
  See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
    """Entry point for launching an IPython kernel.
  </code></pre>
  
  <p>Is there a way to properly perform the column copy without looping over the data frame?</p>