title: Pandas subtracting values of two data frames element wise
body: <p>As part of an object constructor I want to subtract the values of two pandas data frames element wise:</p>
  
  <pre><code>        self.dfload=pd.read_csv(self.name +'/' + 'load.csv')                    
          self.dfload.set_index('snapshot', inplace=True)
  
                           load_1      load_2      load_3      load_4  
  snapshot                                                              
  2018-01-01 00:00:00   68.248569   91.998188   64.988923  139.535086   
  2018-01-01 00:15:00  138.274243  127.186259   80.769227   33.509007   
  2018-01-01 00:30:00  129.824298   56.706114   75.234845  138.610287   
  2018-01-01 00:45:00   51.754610   45.703056   73.060490   36.913774   
  2018-01-01 01:00:00   52.129775  139.315283   67.093788   60.488806
  </code></pre>
  
  <p>and</p>
  
  <pre><code>        self.dfsupply=pd.read_csv(self.name + '/' + 'supply.csv')               
          self.dfsupply.set_index('snapshot', inplace=True)
  
                        supply_1   supply_2   supply_3   supply_4     
  snapshot                                                                     
  2018-01-01 00:00:00  28.448017  45.383377  56.626144  40.044848    
  2018-01-01 00:15:00  37.534878  29.094980  67.722537  15.002448    
  2018-01-01 00:30:00  46.163805  28.324557  26.322953  23.250904     
  2018-01-01 00:45:00  48.192774  55.049855  72.872200  21.602035     
  2018-01-01 01:00:00  60.499436  53.698329  74.674572  42.425620
  </code></pre>
  
  <p>by</p>
  
  <pre><code>self.dfresidualLoad=self.dfsupply.subtract(self.dfload, axis='column')
  </code></pre>
  
  <p>The result is NaN for each element and the concatenation of the two dfs:</p>
  
  <pre><code>                         load_1  load_2  load_3  ...  supply_1  supply_2 ...
  snapshot                                                                        
  2018-01-01 00:00:00     NaN     NaN     NaN     NaN     NaN     NaN
  .
  .         
  </code></pre>
  
  <p>There is no issue by subtracting single columns from each other. Unfortunately this is not the ideal solution since I want to keep the number of columns undefined.</p>