A QGIS field calculator expression to fill down a list with non null value from a column

Fill_down.py

from qgis.core import *
from qgis.gui import *
mem = 'NULL'

@qgsfunction(args='auto', group='Custom', handlesnull=True)
def fill_down(value1, feature, parent):
    """
    Restituisce un valore dal campo specificato,
    dove il valore del campo della riga precedente
    viene propagato ai campi con valori Null successivi
    <p>    
    Returns a value from the specified field,
    where the value of the field of the previous row
    it is propagated to subsequent null fields
    <h2>Example usage:</h2>
    <ul>
      <li>fill_down("Dato") -> Fill_Data</li>
      <table>
        <thead>
        <tr>
        <th>Dato</th>
        <th>Fill_Data</th>
        </tr>
        </thead>
        <tbody>
        <tr>
        <td style="text-align:right">a</td>
        <td style="text-align:right">a</td>
        </tr>
        <tr>
        <td>&nbsp;</td>
        <td style="text-align:right">a</td>
        </tr>
        <tr>
        <td>&nbsp;</td>
        <td style="text-align:right">a</td>
        </tr>
        <tr>
        <td style="text-align:right">b</td>
        <td style="text-align:right">b</td>
        </tr>
        <tr>
        <td></td>
        <td style="text-align:right">b</td>
        </tr>
        </tbody>
        </table>
            </ul>
        
        Opera in base all'id dei record
        <p>
        Operate based on the ID of the records
    """
    global mem
    res = str(value1)
    if res in ('NULL',''):
        res = mem
    else:
        mem = str(value1)
    return res