import openpyxl

## initializing the xlsx
xlsx = openpyxl.Workbook()

## creating an active sheet to enter data
sheet = xlsx.active

## creating data to append
data = [
         [1, 2, 3],
         [4, 5, 6],
         [7, 8, 9],
         [10, 11, 12]
       ]

## appending row by row to the sheet
for row in data:
    ## append method is used to append the data to a cell
    sheet.append(row)

## saving the xlsx file using 'save' method
xlsx.save('appending.xlsx')