title: Python Pandas replace NaN in one column with value from corresponding row of second column
body: <p>I am working with this Pandas DataFrame in Python.</p>
  
  <pre><code>File    heat    Farheit Temp_Rating
     1    YesQ         75         N/A
     1    NoR         115         N/A
     1    YesA         63         N/A
     1    NoT          83          41
     1    NoY         100          80
     1    YesZ         56          12
     2    YesQ        111         N/A
     2    NoR          60         N/A
     2    YesA         19         N/A
     2    NoT         106          77
     2    NoY          45          21
     2    YesZ         40          54
     3    YesQ         84         N/A
     3    NoR          67         N/A
     3    YesA         94         N/A
     3    NoT          68          39
     3    NoY          63          46
     3    YesZ         34          81
  </code></pre>
  
  <p>I need to replace all NaNs in the <code>Temp_Rating</code> column with the value from the <code>Farheit</code> column.</p>
  
  <p>This is what I need:</p>
  
  <pre><code>File        heat    Temp_Rating
     1        YesQ             75
     1         NoR            115
     1        YesA             63
     1        YesQ             41
     1         NoR             80
     1        YesA             12
     2        YesQ            111
     2         NoR             60
     2        YesA             19
     2         NoT             77
     2         NoY             21
     2        YesZ             54
     3        YesQ             84
     3         NoR             67
     3        YesA             94
     3         NoT             39
     3         NoY             46
     3        YesZ             81
  </code></pre>
  
  <p>If I do a Boolean selection, I can pick out only one of these columns at a time. The problem is if I then try to join them, I am not able to do this while preserving the correct order.</p>
  
  <p>How can I only find <code>Temp_Rating</code> rows with the <code>NaN</code>s and replace them with the value in the same row of the <code>Farheit</code> column?</p>