It seems that the unique
constraint is not natively supported in GAE, although one can enforce unique check before putting an object to store.
But tha
You can achieve a "unique constraint" in app engine's data store as described here. The example is presented in Java, but you should be able to apply the same concepts in python.
No, you still cannot specify unique during schema creation.
This issue is located here in case you want to watch it. It doesn't show up on their list of features on deck yet, so it seems like there are other features they will be working on first.