Return from @tornado.gen.engine function without yield?

Question

I'd like to execute or not execute an async query based on the value of a parameter. If the parameter is True, the query shouldn't be executed.

I have a method like this:

@tornado.gen.engine
def retrieveSomeData(self, feelingLucky, callback):
    if feelingLucky:
        return      # <-- doesn't work, function never returns!
    else:
        response = yield tornado.gen.Task(queryFunction, param1....)
        callback(response)

How can I make the feelingLucky branch work?

The only thing I can think of is raising an exception and catching it in the caller. But that's very ugly. Or, if there was such a thing as a null task...

(Python 2.7, Tornado 3.2)

Answer 1

Better to use the modern gen.coroutine instead of the obsolete gen.engine. That makes this sort of conditional logic simple and natural:

@tornado.gen.coroutine
def retrieveSomeData(self, feelingLucky):
    if feelingLucky:
        return
    else:
        response = yield Tornado.gen.Task(queryFunction, param1....)
        raise gen.Return(response)

If you convert queryFunction to coroutine style too, you get:

@tornado.gen.coroutine
def retrieveSomeData(self, feelingLucky):
    if feelingLucky:
        return
    else:
        response = yield queryFunction(param1....)
        raise gen.Return(response)