Those extent objects can be a bit tricky, I might amend it to something like:
#create a copy of extent object to modify
new_extent = objDes.extent
#set values for extent object
new_extent.XMin = objDes.extent.XMin
new_extent.XMax = objDes.extent.XMax
new_extent.YMin = objDes.extent.YMin
new_extent.YMax = objDes.extent.YMax
return new_extent
No idea if that will work as I struggle with these also. You may just want to run the 'Feature envelope to polygon' tool on the data then use that polygon as the extent if not.