title: Python Pandas Match Vlookup columns based on header values
body: <p>I have the following dataframe df: </p>
  
  <pre><code>Customer_ID | 2015 | 2016 |2017 | Year_joined_mailing
  ABC            5      6     10     2015
  BCD            6      7     3      2016        
  DEF            10     4     5      2017
  GHI            8      7     10     2016
  </code></pre>
  
  <p>I would like to look up the value of the customer in the year they joined the mailing list and save it in a new column. </p>
  
  <p>Output would be: </p>
  
  <pre><code>Customer_ID | 2015 | 2016 |2017 | Year_joined_mailing | Purchases_1st_year
  ABC            5      6     10     2015                       5
  BCD            6      7     3      2016                       7       
  DEF            10     4     5      2017                       5
  GHI            8      9     10     2016                       9
  </code></pre>
  
  <p>I have found some solutions for match vlookup in python, but none that would use the headers of other columns. </p>