title: Pandas missing value; with fflill and add comment
body: <p>following through pandas documentation for <code>df.fillna(method=&quot;ffill&quot;)</code>, <a href="https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.fillna.html" rel="nofollow noreferrer">here</a>.
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  <pre><code> df_1 = pd.DataFrame([['2021-01-01', 'Supp_1', 'Product_1', 1],
                         ['2021-02-01', 'Supp_1', 'Product_1', ''],
                         ['2021-03-01','Supp_1', 'Product_1', np.nan],
                         ['2021-04-01', 'Supp_1', 'Product_1', 1.25],
                ['2021-01-01', 'Supp_1', 'Product_2', 1.5],
                         ['2021-02-01', 'Supp_1', 'Product_2', ''],
                         ['2021-03-01','Supp_1', 'Product_2', np.nan],
                         ['2021-04-01', 'Supp_1', 'Product_2', 1.75]],
                        columns=['Date','Supplier','Product','Cost'])
  
        Date     Supplier Product     Cost
  0   2021-01-01  Supp_1  Product_1   1
  1   2021-02-01  Supp_1  Product_1   
  2   2021-03-01  Supp_1  Product_1   NaN
  3   2021-04-01  Supp_1  Product_1   1.25
  4   2021-01-01  Supp_1  Product_2   1.5
  5   2021-02-01  Supp_1  Product_2   
  6   2021-03-01  Supp_1  Product_2   NaN
  7   2021-04-01  Supp_1  Product_2   1.75
  </code></pre>
  <p>Expected df_2,</p>
  <pre><code>       Date     Supplier Product Cost   Cost_Assumption
  0   2021-01-01  Supp_1  Product_1   1.00    Actual
  1   2021-02-01  Supp_1  Product_1   1.00    Cost per 2021-01-01
  2   2021-03-01  Supp_1  Product_1   1.00    Cost per 2021-01-01
  3   2021-04-01  Supp_1  Product_1   1.25    Actual
  4   2021-01-01  Supp_1  Product_2   1.50    Actual
  5   2021-02-01  Supp_1  Product_2   1.50    Cost per 2021-01-01
  6   2021-03-01  Supp_1  Product_2   1.50    Cost per 2021-01-01
  7   2021-04-01  Supp_1  Product_2   1.75    Actual
  </code></pre>