Handling Google App Engine datastore NoneType errors

Question

I wish to handle NoneType errors from the Google App Engine ndb Datastore.

Here is my code:

def post(self):
    # get the email and password from the login form
    global u_qry
    email = self.request.get("email")
    password = self.request.get("password")

    try:
        u_qry = user_db.User.get_by_email(email)
    except BaseException, e:
        self.response.out.write(e, u_qry)
    else:
        self.response.out.write("User Recognised")
        self.response.out.write(u_qry)

When the user does not exist, the except clause is not catching any error.

Output:
User Recognised None

why is the except clause not catching this error? What exception can be used to catch this error?

I realise I can use if/else statements on this, but would like to learn how to use the try/except clause in GAE.

Thanks.

Answer 1

While you're not showing the way the User.get_by_email() function is implemented I suspect it is simply based on a query. And returning an empty result from a query is a perfectly valid situation - no existing datastore entity matches the query. It's not an exception.

If you intend to consider such case an exception you have to raise one yourself inside User.get_by_email() and in that case you'd be able to catch it in your post() function.

Alternatively you can directly check u_qry for a None value.