Debugging with asyncio

There are several useful debugging features built into asyncio.

First, the event loop uses logging to emit status messages as it runs. Some of these are available if logging is enabled in an application. Others can be turned on by telling the loop to emit more debugging messages. Call set_debug() passing a boolean value indicating whether or not debugging should be enabled.

Because applications built on asyncio are highly sensitive to greedy coroutines failing to yield control, there is support for detecting slow callbacks built into the event loop. Turn it on by enabling debugging, and control the definition of “slow” by setting the slow_callback_duration property of the loop to the number of seconds after which a warning should be emitted.

Finally, if an application using asyncio exits without cleaning up some of the coroutines or other resources, that may mean there is a logic error preventing some of the application code from running. Enabling ResourceWarning warnings causes these cases to be reported when the program exits.

asyncio_debug.py
import argparse
import asyncio
import logging
import sys
import time
import warnings

parser = argparse.ArgumentParser('debugging asyncio')
parser.add_argument(
    '-v',
    dest='verbose',
    default=False,
    action='store_true',
)
args = parser.parse_args()

logging.basicConfig(
    level=logging.DEBUG,
    format='%(levelname)7s: %(message)s',
    stream=sys.stderr,
)
LOG = logging.getLogger('')


async def inner():
    LOG.info('inner starting')
    # Use a blocking sleep to simulate
    # doing work inside the function.
    time.sleep(0.1)
    LOG.info('inner completed')


async def outer(loop):
    LOG.info('outer starting')
    await asyncio.ensure_future(loop.create_task(inner()))
    LOG.info('outer completed')


event_loop = asyncio.get_event_loop()
if args.verbose:
    LOG.info('enabling debugging')

    # Enable debugging
    event_loop.set_debug(True)

    # Make the threshold for "slow" tasks very very small for
    # illustration. The default is 0.1, or 100 milliseconds.
    event_loop.slow_callback_duration = 0.001

    # Report all mistakes managing asynchronous resources.
    warnings.simplefilter('always', ResourceWarning)

LOG.info('entering event loop')
event_loop.run_until_complete(outer(event_loop))

When run without debugging enabled, everything looks fine with this application.

$ python3 asyncio_debug.py

  DEBUG: Using selector: KqueueSelector
   INFO: entering event loop
   INFO: outer starting
   INFO: inner starting
   INFO: inner completed
   INFO: outer completed

Turning on debugging exposes some of the issues it has, including the fact that although inner() finishes it takes more time to do so than the slow_callback_duration that has been set and that the event loop is not being properly closed when the program exits.

$ python3 asyncio_debug.py -v

  DEBUG: Using selector: KqueueSelector
   INFO: enabling debugging
   INFO: entering event loop
   INFO: outer starting
   INFO: inner starting
   INFO: inner completed
WARNING: Executing <Task finished coro=<inner() done, defined at
asyncio_debug.py:34> result=None created at asyncio_debug.py:44>
took 0.102 seconds
   INFO: outer completed
.../lib/python3.5/asyncio/base_events.py:429: ResourceWarning:
unclosed event loop <_UnixSelectorEventLoop running=False
closed=False debug=True>
  DEBUG: Close <_UnixSelectorEventLoop running=False
closed=False debug=True>