How to work with Futures.

[wsantos]

i'm tring to make some queries on my model to clean my RequestHandlers i can get it working but i dont know if is the right way

Query

class Rates():

    def latest_rates(self, db):

        future = Future()

        def handle_response(response, error):

            import ipdb; ipdb.set_trace()

            if response.error:

                future.set_exception(response.error)

            else:

                future.set_result(response)

        db.rates.find_one({}, callback=handle_response)

        return future

RequestHandler

class MainHandler(RatingHandler):

    """"""

    @tornado.web.asynchronous

    @gen.coroutine

    def get(self):

        rate = model.Rate()

        t = yield rate.latest_rates(self.db)

it is working, but i really need to create a handle_response for every method in my model ? and how i can chain another query inside latest_rates ?

regrads.

[Ben Darnell]

In general, I would recommend using one of Tornado's two decorators @gen.coroutine and @tornado.concurrent.return_future instead of creating a Future manually. Your latest_rates function would then look like one of these:

  @gen.coroutine

  def latest_rates(self, db):

    response = yield gen.Task(db.rates.find_one, {})

    if response.error:

      raise response.error

    else:

      raise gen.Return(response)

  @return_future

  def latest_rates(self, db, callback):

    def handle_response(response, error):

      if response.error:

        raise response.error

      else:

        callback(response)

    db.rates.find_one({}, callback=handle_response)

-Ben

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[Waldecir Santos]

Nice, i like the @return_future, but exisits a way to define the handle_response to be used in more than one function ?

Grato,

Waldecir

[Ben Darnell]

Of course.  The function can be defined wherever you want.  You can use a functools.partial or maybe a bound self pointer to get any state you need (the callback in this example) into handle_response.

-Ben

[Waldecir Santos]

like this  ?

class Rate(Model):

    def handle_response(self, response, error, callback):

        assert callback, iscallable

        if error:

            raise error

        else:

            callback(response)

    @return_future

    def latest_rates(self, db, callback):

        db.rates.find_one({}, callback=functools.partial(self.handle_response, callback=callback))

is that right ? its running here, but i dont know if i can have bad performance impact.

 

Grato,

Waldecir

[Ben Darnell]

Yes, that looks fine.

-Ben

[Waldecir Santos]

Thanks Ben, now i need more one  advice, latest_rates returns something like this

{

[ 

user_id : ObjectID(XXXXXXXXXX), 

rate: { xxx: yyyyy, zzzz: ccccc}
]
 

 I need to loop over the find in latest_rates to query user and inject/join the user

Grato,

Waldecir

[Waldecir Santos]

Well i can do preatty much, but i cant get latest_rates to return a value

    @return_future

    def get(self, db, id, callback):

        db.users.find_one({"_id": id}, callback=functools.partial(self.handle_response, callback=callback))

    @return_future

    @gen.coroutine

    def latest_rates(self, db, callback):

        rates = []

        cursor = db.rates.find()

        while (yield cursor.fetch_next):

            m_rate = cursor.next_object()

            m_user = yield self.get(db, m_rate['rater_id'])

            rates.append("xxx")

        callback(rates)

Im getting a exception 

    Traceback (most recent call last):

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/web.py", line 1055, in _stack_context_handle_exception

        raise_exc_info((type, value, traceback))

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/stack_context.py", line 239, in wrapped

        callback(*args, **kwargs)

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/web.py", line 1187, in future_complete

        f.result()

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/concurrent.py", line 129, in result

        raise_exc_info(self.__exc_info)

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/stack_context.py", line 270, in _nested

        if exit(*exc):

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/stack_context.py", line 180, in __exit__

        return self.exception_handler(type, value, traceback)

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/concurrent.py", line 211, in handle_error

        future.set_exc_info((typ, value, tb))

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/concurrent.py", line 125, in set_exc_info

        self.set_exception(exc_info[1])

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/concurrent.py", line 87, in set_exception

        self._set_done()

      File "/Users/wsantos/.virtualenvs/rating/lib/python2.7/site-packages/tornado/concurrent.py", line 95, in _set_done

        for cb in self._callbacks:

    TypeError: 'NoneType' object is not iterable

if i remove @return_futures and callback from latest_rates i can print all fine, i dont know to return now., and i dont know why documentation say to not user coroutine and return_future together, can you explain Ben, i think would be good to explain too in docs. 

Grato,

Waldecir

[Ben Darnell]

This error is not very helpful but what it means is that there was an error (which is not being printed) after the Future was already set as done.

 

if i remove @return_futures and callback from latest_rates i can print all fine, i dont know to return now., and i dont know why documentation say to not user coroutine and return_future together, can you explain Ben, i think would be good to explain too in docs. 

Using gen.coroutine and return_future at the same time should work (I think), but it's redundant because @gen.coroutine already returns a Future.  The docs for gen.coroutine say "@gen.coroutine is similar to the combination of @return_future and @gen.engine.".  In most cases I would recommend just using @gen.coroutine; only use @return_future if you prefer to have the callback argument exposed explicitly.

To return from a function using @gen.coroutine, raise gen.Return(value).  (in Python 3.3 it is possible to just use the return statement normally)

The problem you're seeing with the two decorators together is that @gen.coroutine hides the callback from the decorated function, and requires that it be passed by keyword instead of by position.  I think if you removed the callback argument and used gen.Return it would work with both decorators (but again there is no benefit to doing so).

-Ben

[Waldecir Santos]

Nice, i can get it working like this

    @gen.coroutine

    def latest_rates(self, db):

        rates = []

        cursor = db.rates.find()

        while (yield cursor.fetch_next):

            m_rate = cursor.next_object()

            m_user = yield self.get(db, m_rate['rater_id'])

            rates.append("xxx")

        raise gen.Return(rates)

But im counfused, is this the "right way/patterns" to follow  ? can i return value without the raise, and i dont get when you say " (but again there is no benefit to doing so)."

Grato,

Waldecir