title: Divide columns by another columns with NaN in pandas
body: <p>I am a newbie in python.</p>
  
  <p>I have a dataframe as shown below.</p>
  
  <pre><code>Price        Area
  80           4
  90           10
  20           NaN
  60           12
  50           NaN
  </code></pre>
  
  <p>From the above I would like to prepare below DF.</p>
  
  <p>Expected Output:</p>
  
  <pre><code>Price        Area    Price_Per_Unit_Area
  80           4       20
  90           10      9
  20           NaN     NaN
  60           12      5
  50           NaN     NaN
  </code></pre>
  
  <p>I tried below code</p>
  
  <pre><code>df['Price_Per_Unit_Area'] = df['Price']/df['Area']
  </code></pre>
  
  <p>I got full NaN as shown below</p>
  
  <pre><code>    Price        Area    Price_Per_Unit_Area
      80           4       NaN
      90           10      NaN
      20           NaN     NaN
      60           12      Nan
      50           NaN     NaN
  </code></pre>
  
  <p>Error are:</p>
  
  <pre><code>/Users/d.ali/pythenvda/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>