title: Iteratively update the values of a dataframe with another one
body: <p>I have a main df:</p>
  
  <pre><code>print(df)
  
     item            dt_op     
  0  product_1     2019-01-08   
  1  product_2     2019-02-08    
  2  product_1     2019-01-08        
  ...
  </code></pre>
  
  <p>and a subset of the first one, that contains only one product and two extra columns:</p>
  
  <pre><code>print(df_1)
  
     item            dt_op        DQN_Pred  DQN_Inv
  0  product_1     2019-01-08         6      7.0
  2  product_1     2019-01-08         2      2.0
  ...
  </code></pre>
  
  <p>That I am iteratively creating, with a for loop (hence, <code>df_1 = df.loc[df.item == i] for i in items</code>).</p>
  
  <p>I would like to merge <code>df_1</code> and <code>df</code>, at every step of the iteration, hence updating df with the two extra columns.</p>
  
  <pre><code>print(final_df)
  
     item            dt_op      DQN_Pred  DQN_Inv
  0  product_1     2019-01-08       6      7.0  
  1  product_2     2019-02-08      nan      nan
  2  product_1     2019-01-08       2      2.0     
  ...
  </code></pre>
  
  <p>and update the nan at the second step of the for loop, in which <code>df_1</code> only contains <code>product_2</code>.</p>
  
  <p>How can I do it?</p>