Google App Engine (Python): Allow entity 'previewing before 'submit'
It aint that difficult. Abstract: 

[User]-> Posts [Data] to the [EntityCreatorPreviewHandler]
[EntityCreatorPreviewHandler]-> Recieves the data and creates the entity eg: book = Book(title='Test'). 
[EntityCreatorPreviewHandler]-> Templates the html and basically shows the entity with all it's attributes etc.
[EntityCreatorPreviewHandler]-> Also hides the initial [Data] in a hidden post form
[User]-> Accepts save after preview and as soon as the save button is pressed the hidden form is submitted to a EntitySaveHandler
[EntitySaveHandler] saves the data
I'd like users to create an entity, and preview it, before saving it in the datastore.
For example:

User completes entity form, then clicks "preview".
Forwarded to an entity 'preview' page which allows the user to "submit" and save the entity in the datastore, or "go back" to edit the entity.

How can I achieve this?