Tornado firing vs yielding

README.md

Yielding a list of tasks in Tornado 4.0

Note: This behavior has changed in Tornado 4.1

Executing python yield_all.py will produce results similar to:

$ python run_all.py
one is sleeping
two is sleeping
three is sleeping
four is sleeping
Beginning yield
four is raising exception!
two is raising exception!
three is raising exception!
one is raising exception!
Traceback (most recent call last):
  File "run_all.py", line 48, in <module>
    ioloop.IOLoop.instance().run_sync(raise_many)
  File "run_all.py", line 30, in raise_many
    yield tasks
  File "run_all.py", line 18, in raise_exception
    raise Exception('Bad things from %s' % name)
Exception: Bad things from one

Note two important things:

1. "Beginning yield" starts after the sleeping has been initiated.
2. The traceback you see is always from one and not from a random name.

yield_all.py

import random
import time

from tornado import gen
from tornado import ioloop

@gen.coroutine
def tornado_sleep(seconds=1.0):
    yield gen.Task(ioloop.IOLoop.current().add_timeout,
                   time.time() + seconds)

@gen.coroutine
def raise_exception(name):
    print "%s is sleeping" % name
    seconds = random.random()
    yield tornado_sleep(seconds)
    print "%s is raising exception!" % name
    raise Exception('Bad things from %s' % name)

@gen.coroutine
def raise_many():

    tasks = [raise_exception('one'),
             raise_exception('two'),
             raise_exception('three'),
             raise_exception('four')]

    errors = []
    print "Beginning yield"
    yield tasks


def print_tidy_exception(e):
    # Skip traceback that involves site-packages.
    import traceback
    trace_lines = traceback.format_exc(e).splitlines()
    skip_next = False
    for l in trace_lines:
        if 'tornado' in l:
            skip_next = True
            continue
        if not skip_next:
            print(l)
        skip_next = False

if __name__ == '__main__':
    try:
        ioloop.IOLoop.instance().run_sync(raise_many)
    except Exception as e:
        print_tidy_exception(e)