Run async function after another one

[Corentin Leruth]

Hello,

I would like to run 2 async function but the second after the first one has finished. For exemple:


do_something
Async_function1(param1, param2,..) Note that I don't yield this function because I don't need his result to continue
Async_function2(other_param1, other_param2, ..) Another async function who don't need the result of the first to run. But this function must be launched after the first is finished. I don't yield his result because I don't need this to continue
do_something2..

If I yield the 2 async_function, it's work but "do_something2" have to wait for their results and it's not necessary...

I tried with IOloop.add_future but the second async function don't need the Future from the first one. Each async_function have different param...


It's possible to achieve this behaviour ?

Thank you and sorry for my bad english :P

[A. Jesse Jiryu Davis]

You could just wrap async_function_1 and async_function_2 in another coroutine, and run it without yielding:

from tornado.ioloop import IOLoop

from tornado import gen

@gen.coroutine

def async_function_1():

    yield gen.sleep(1)

    print('async_function_1 done')

@gen.coroutine

def async_function_2():

    yield gen.sleep(1)

    print('async_function_2 done')

@gen.coroutine

def task():

    yield async_function_1()

    yield async_function_2()

    IOLoop.current().stop()  # Only for demonstration.

@gen.coroutine

def do_something_2():

    print('do_something_2 done')

@gen.coroutine

def do_something():

    task()

    yield do_something_2()

    print('do_something done')

do_something()

IOLoop.current().start()

To ensure you handle any exception thrown by task() you might do:

def handle_err(future):

    try:

        future.result()

    except Exception as e:

        # Log exception....

        pass

@gen.coroutine

def do_something():

    future = task()

    IOLoop.current().add_future(future, handle_err)

    yield do_something_2()

    print('do_something done')

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[Ben Darnell]

It's better to use IOLoop.spawn_callback than to simply call a coroutine without yielding it - this will avoid certain edge cases that can occur in exception handling, and gives you a default handler to log all errors (so you don't need to define a handle_err wrapper unless you want to do something else).

In other words:

  @gen.coroutine

  def do_something():

    # Note that there are no parens after 'task'; you're passing the 

    # function to spawn_callback, not calling it.

    IOLoop.current().spawn_callback(task)

    yield do_something2()