Query Site (http://onlinehelp.tableau.com/current/api/rest_api/en-us/help.htm#REST/rest_api_ref.htm#Query_Site%3FTocPath%3DAPI%2520Reference%7C_____56) has support for getting a site by name and content url:

GET /api/api-version/sites/site-name?key=name
GET /api/api-version/sites/content-url?key=contentUrl

Implement support in TSC for these (e.g. get_by_name(self, site_name))