How to use python to collapse rows in already existing excel sheet?

Question

An example of what I'm trying to do is this

http://xlsxwriter.readthedocs.org/en/latest/example_outline2.html

Xlsxwriter doesn't allow you to change an already existing worksheet though.

I'm looking for a python library that will let me collapse excel rows in an already existing worksheet.

I've tried looking into xlrd but haven't had any luck. Does anyone know a method for doing this?