The documentation only talks about how to do it from ruby.
Get the connection string settings by running heroku config on the command line after installed the add-on to your heroku app.
There will be an entry with the key MONGOLAB_URI in this form:
MONGOLAB_URI => mongodb://user:pass@xxx.mongolab.com:27707/db
Simply the info from the uri in python by creating a connection from the uri string.
How can I use the mongolab add-on to Heroku from python?