title: I have two dataframes and I need to compare them to each other and calculate a measure
body: <p>I have two pandas dataframes in python and need to do a measure based on date condition</p>
  <p>DF1 has a column named VALUE that I need to divide based on column EXCHANGE from DF2</p>
  <pre><code>
  DF1: 
  
     MONTH     VALUE
  2021-01-01    50
  2021-01-01    75
  2021-01-01    100
  2021-03-01    150
  2021-04-01    100
  2021-03-01    150
  2021-03-01    150 
  
  DF2: 
      MONTH     EXCHANGE
  2021-01-01     4
  2021-02-01     4
  2021-03-01     2
  2021-04-01     10
  
  I NEED TO TRANSFORM THE DF1 INTO:
  
     MONTH               VALUE
  2021-01-01              12.5 --&gt; 50/4
  2021-01-01              18.75 --&gt; 75 (PREVIOUS VALUE) / 4 (EXCHANGE VALUE)
  2021-01-01              25
  2021-03-01              75
  2021-04-01              10   --&gt; 100/10                     
  2021-03-01              75
  2021-03-01              75   
  </code></pre>
  <p>Thank you in advance.</p>