Selecting by id in Google's Datastore API

While developing a Google App, I have found some difficulty while trying to retrieve data by id. The method is very simple, and below is the Phyton code:

objectname = ModelName.get_by_id(longIntegerId)

This is how to select by id from Google's Datastore API

Thanks to: http://www.thinkvitamin.com/features/webapps/developing-with-google-app-engine-part-i