Edit existing excel workbooks and sheets with xlrd

Question

In the documentation for xlrd and xlwt I have learned the following:

How to read from existing work-books/sheets:

from xlrd import open_workbook
wb = open_workbook("ex.xls")
s = wb.sheet_by_index(0)
print s.cell(0,0).value
#Prints contents of cell at location a1 in the first sheet in the document called ex.xls

How to create new work-books/sheets:

from xlwt import Workbook
wb = Workbook()
Sheet1 = wb.add_sheet('Sheet1')
Sheet1.write(0,0,'Hello')
wb.save('ex.xls')
#Creates a document called ex.xls with a worksheet called "Sheet1" and writes "Hello" to the cell located at a1

What I want to do now is to open an existing worksheet, in an existing workbook and write to that sheet.

I have tried something like:

from xlwt import open_workbook
wb = open_workbook("ex.xls")
s = wb.sheet_by_index(0)
print s.cell(0,0).value

but open_workbook is only part of the xlrd module, not xlwt.

Any ideas?

Edit1: After Olivers suggestion I looked into xlutils and tried the following:

from xlrd import open_workbook
from xlwt import Workbook
from xlutils.copy import copy

wb = open_workbook("names.xls")
s = wb.get_sheet(0)
s.write(0,0,'A1')
wb.save('names.xls')

This however, gives me the following error message:

File "C:\Python27\lib\site-packages\xlrd\book.py", line 655, in get_sheet
raise XLRDError("Can't load sheets after releasing resources.")
xlrd.biffh.XLRDError: Can't load sheets after releasing resources.

Edit 2: The error message was due to improper use of the get_sheet function. Finally found out how to use it:

from xlrd import open_workbook
from xlwt import Workbook
from xlutils.copy import copy

rb = open_workbook("names.xls")
wb = copy(rb)

s = wb.get_sheet(0)
s.write(0,0,'A1')
wb.save('names.xls')

Answer 1

Here's another way of doing the code above using the openpyxl module that's compatible with xlsx. From what I've seen so far, it also keeps formatting.

from openpyxl import load_workbook
wb = load_workbook('names.xlsx')
ws = wb['SheetName']
ws['A1'] = 'A1'
wb.save('names.xlsx')

Answer 2

As I wrote in the edits of the op, to edit existing excel documents you must use the xlutils module (Thanks Oliver)

Here is the proper way to do it:

#xlrd, xlutils and xlwt modules need to be installed.  
#Can be done via pip install <module>
from xlrd import open_workbook
from xlutils.copy import copy

rb = open_workbook("names.xls")
wb = copy(rb)

s = wb.get_sheet(0)
s.write(0,0,'A1')
wb.save('names.xls')

This replaces the contents of the cell located at a1 in the first sheet of "names.xls" with the text "a1", and then saves the document.