To export google spreadsheet's single worksheet to CSV, integer worksheet index(GID) is required to be passed.
https://spreadsheets.google.com/feeds/download/spreadsheets/Export?key=%s&gid=%d&exportFormat=csv
But, where are those informations? With gdata.spreadsheets.client, I could find some string id for worksheet like "oc6, ocv, odf".
client = gdata.spreadsheets.client.SpreadsheetsClient()
feed = client.GetWorksheets(spreadsheet, auth_token=auth_token)
And it returns below atom XML. (part of it)
<entry gd:etag=""URJFCB1NQSt7ImBoXhU."">
<id>https://spreadsheets.google.com/feeds/worksheets/0AvhN_YU3r5e9dGpTWGx3UVU3MTczaXJuNEFKQjMwN2c/ocw</id>
<updated>2012-06-21T08:19:46.587Z</updated>
<app:edited xmlns:app="http://www.w3.org/2007/app">2012-06-21T08:19:46.587Z</app:edited>
<category scheme="http://schemas.google.com/spreadsheets/2006" term="http://schemas.google.com/spreadsheets/2006#worksheet"/>
<title>AchievementType</title>
<content type="application/atom+xml;type=feed" src="https://spreadsheets.google.com/feeds/list/0AvhN_YU3r5e9dGpTWGx3UVU3MTczaXJuNEFKQjMwN2c/ocw/private/full"/>
<link rel="http://schemas.google.com/spreadsheets/2006#cellsfeed" type="application/atom+xml" href="https://spreadsheets.google.com/feeds/cells/0AvhN_YU3r5e9dGpTWGx3UVU3MTczaXJuNEFKQjMwN2c/ocw/private/full"/>
<link rel="http://schemas.google.com/visualization/2008#visualizationApi" type="application/atom+xml" href="https://spreadsheets.google.com/tq?key=0AvhN_YU3r5e9dGpTWGx3UVU3MTczaXJuNEFKQjMwN2c&sheet=ocw"/>
<link rel="self" type="application/atom+xml" href="https://spreadsheets.google.com/feeds/worksheets/0AvhN_YU3r5e9d