After Looking into the problem I found a solution using the xlwt
library to write the data on a virtual workbook and the xlutils
library to save it and thus make the virtual workbook into an actual .xls file.
import xlrd
import xlwt
from xlutils.copy import copy
import os.path
rb = xlrd.open_workbook('my_workbook.xls',formatting_info=True)
r_sheet = rb.sheet_by_index(0)
wb = copy(rb)
sheet = wb.get_sheet(0)
sheet.write(5,2,"string")
wb.save('my_workbook.xls')