How to do this kind of session related task in App Engine Python?
You'd use the datastore to create a union as an entity class, with a description and a name.  If your image is small you can store it in your entity, if it's large, you may store it in the blobstore and store a link to it inside your entity.
You can use the python User API for authentication.  You don't really need any special session work if you're using the User API.
First than all, I don't even know if this is a session related question. But I could not think a better way to describe it in the title.
I'm developing a web application for registered users so they can create and manage trade unions.
A user can create several unions. Each union can store an image, a description and a name.
The index page shows the list of unions created by the currently registered user.
When the user clicks on a union from the list, all the pages of the application must show
in they headers the corresponding name and image stored for that union.
Also, all the options of the application must refer to the currently selected union. 
That is the process for every selected union.
How could I do this on App Engine Python? What technique could I use? Is it something
related to sessions? I do the authentication process with the Gmail service.
I hope I explained myself clearly.
Thanks in advance!