'int editLength ( float increment, string spool_name, int extruder )
Add the given \'increment\' amount to the length of filament of the
given spool. Spool can be specified either by name or by the extruder
it is loaded to.'

def editLength(self, increment, spool_name=None, extruder=(-1)):

    if (spool_name != None):
        spool_item = self._findByColumn(spool_name, 0)
    elif (extruder != (-1)):
        spool_item = self._findByColumn(extruder, 2)
    else:
        return (-1)
    if (spool_item == (-1)):
        return (-2)
    length = (spool_item[1] + increment)
    self.remove(spool_item[0])
    self.add(spool_item[0], length)
    if (spool_item[2] > (-1)):
        self.load(spool_item[0], spool_item[2])
    self._save()
    return 0
