title: Conditional enrichment in Pandas dataframes
body: <p>I have two Pandas Dataframes, which consist of the same columns and are partly filled with similar data. Each row in the dataframe has a unique id.</p>
  <pre><code>df1
        ID        A            B      C      D 
  0    23149       -112.2      15     454   456 
  1    89799      -221.84      12     454   
  2    11589       339.93      48           564
  3    67867       -855.5      13         
  4    7898         68.4       90         
  5    789         45.1        10       
  6    456          4564       12       
  </code></pre>
  <pre><code>df2 (before)
        ID        A            B      C     D 
  0    23149       -112.2      15         
  1    23154         68.4      90       
  2    11589       339.93      48         
  3    97897       -855.5      13         
  4    89799      -221.84      12  
  </code></pre>
  <p>I want to enrich the columns C and D in df2 with values from df1.</p>
  <ul>
  <li>ID, A and B should not be changed in df2</li>
  <li>If an ID exists in both dataframes the values for C and D should be added from df1.</li>
  </ul>
  <p>In my example I expect a dataframe similar to this.</p>
  <pre><code>df2 (afterwards)
        ID        A            B      C     D 
  0    23149       -112.2      15    454   456  
  1    23154         68.4      90          
  2    11589       339.93      48          564
  3    97897       -855.5      13    
  4    89799      -221.84      12    454     
  </code></pre>
  <p>How can I implement this conditional filling?</p>