
Develop with asyncio
********************

Asynchronous programming is different than classical "sequential"
programming. This page lists common traps and explains how to avoid
them.


Debug mode of asyncio
=====================

To enable the debug mode globally, set the environment variable
"PYTHONASYNCIODEBUG" to "1". To see debug traces, set the log level of
the *asyncio logger* to "logging.DEBUG".  The simplest configuration
is:

   import logging
   # ...
   logging.basicConfig(level=logging.DEBUG)

Examples of effects of the debug mode:

* Log *coroutines defined but never "yielded from"*

* "call_soon()" and "call_at()" methods raise an exception if they
  are called from the wrong thread.

* Log the execution time of the selector

* Log callbacks taking more than 100 ms to be executed. The
  "BaseEventLoop.slow_callback_duration" attribute is the minimum
  duration in seconds of "slow" callbacks.

See also: The "BaseEventLoop.set_debug()" method and the *asyncio
  logger*.


Concurrency and multithreading
==============================

An event loop runs in a thread and executes all callbacks and tasks in
the same thread. While a task is running in the event loop, no other
task is running in the same thread. But when the task uses "yield
from", the task is suspended and the event loop executes the next
task.

To schedule a callback from a different thread, the
"BaseEventLoop.call_soon_threadsafe()" method should be used. Example
to schedule a coroutine from a different thread:

   loop.call_soon_threadsafe(asyncio.async, coro_func())

Most asyncio objects are not thread safe. You should only worry if you
access objects outside the event loop. For example, to cancel a
future, don't call directly its "Future.cancel()" method, but:

   loop.call_soon_threadsafe(fut.cancel)

To handle signals and to execute subprocesses, the event loop must be
run in the main thread.

The "BaseEventLoop.run_in_executor()" method can be used with a thread
pool executor to execute a callback in different thread to not block
the thread of the event loop.

See also: See the *Synchronization primitives* section to
  synchronize tasks.


Handle blocking functions correctly
===================================

Blocking functions should not be called directly. For example, if a
function blocks for 1 second, other tasks are delayed by 1 second
which can have an important impact on reactivity.

For networking and subprocesses, the "asyncio" module provides high-
level APIs like *protocols*.

An executor can be used to run a task in a different thread or even in
a different process, to not block the thread of the event loop. See
the "BaseEventLoop.run_in_executor()" method.

See also: The *Delayed calls* section details how the event loop
  handles time.


Logging
=======

The "asyncio" module logs information with the "logging" module in the
logger "'asyncio'".


Detect coroutine objects never scheduled
========================================

When a coroutine function is called and its result is not passed to
"async()" or to the "BaseEventLoop.create_task()" method: the
execution of the coroutine objet will never be scheduled and it is
probably a bug. *Enable the debug mode of asyncio* to *log a warning*
to detect it.

Example with the bug:

   import asyncio

   @asyncio.coroutine
   def test():
       print("never scheduled")

   test()

Output in debug mode:

   Coroutine test() at test.py:3 was never yielded from
   Coroutine object created at (most recent call last):
     File "test.py", line 7, in <module>
       test()

The fix is to call the "async()" function or the
"BaseEventLoop.create_task()" method with the coroutine object.

See also: *Pending task destroyed*.


Detect exceptions never consumed
================================

Python usually calls "sys.displayhook()" on unhandled exceptions. If
"Future.set_exception()" is called, but the exception is never
consumed, "sys.displayhook()" is not called. Instead, a *a log is
emitted* when the future is deleted by the garbage collector, with the
traceback where the exception was raised.

Example of unhandled exception:

   import asyncio

   @asyncio.coroutine
   def bug():
       raise Exception("not consumed")

   loop = asyncio.get_event_loop()
   asyncio.async(bug())
   loop.run_forever()

Output:

   Task exception was never retrieved
   future: <Task finished bug() done at asyncio/coroutines.py:139 exception=Exception('not consumed',)>
   source_traceback: Object created at (most recent call last):
     File "test.py", line 10, in <module>
       asyncio.async(bug())
     File "asyncio/tasks.py", line 510, in async
       task = loop.create_task(coro_or_future)
   Traceback (most recent call last):
     File "asyncio/tasks.py", line 244, in _step
       result = next(coro)
     File "coroutines.py", line 78, in __next__
       return next(self.gen)
     File "asyncio/coroutines.py", line 141, in coro
       res = func(*args, **kw)
     File "test.py", line 7, in bug
       raise Exception("not consumed")
   Exception: not consumed

*Enable the debug mode of asyncio* to get the traceback where the task
was created.

There are different options to fix this issue. The first option is to
chain to coroutine in another coroutine and use classic try/except:

   @asyncio.coroutine
   def handle_exception():
       try:
           yield from bug()
       except Exception:
           print("exception consumed")

   loop = asyncio.get_event_loop()
   asyncio.async(handle_exception())
   loop.run_forever()

Another option is to use the "BaseEventLoop.run_until_complete()"
function:

   task = asyncio.async(bug())
   try:
       loop.run_until_complete(task)
   except Exception:
       print("exception consumed")

See also the "Future.exception()" method.


Chain correctly coroutines
==========================

When a coroutine function calls other coroutine functions and tasks,
they should be chained explicitly with "yield from". Otherwise, the
execution is not guaranteed to be sequential.

Example with different bugs using "asyncio.sleep()" to simulate slow
operations:

   import asyncio

   @asyncio.coroutine
   def create():
       yield from asyncio.sleep(3.0)
       print("(1) create file")

   @asyncio.coroutine
   def write():
       yield from asyncio.sleep(1.0)
       print("(2) write into file")

   @asyncio.coroutine
   def close():
       print("(3) close file")

   @asyncio.coroutine
   def test():
       asyncio.async(create())
       asyncio.async(write())
       asyncio.async(close())
       yield from asyncio.sleep(2.0)
       loop.stop()

   loop = asyncio.get_event_loop()
   asyncio.async(test())
   loop.run_forever()
   print("Pending tasks at exit: %s" % asyncio.Task.all_tasks(loop))
   loop.close()

Expected output:

   (1) create file
   (2) write into file
   (3) close file
   Pending tasks at exit: set()

Actual output:

   (3) close file
   (2) write into file
   Pending tasks at exit: {<Task pending create() at test.py:7 wait_for=<Future pending cb=[Task._wakeup()]>>}
   Task was destroyed but it is pending!
   task: <Task pending create() done at test.py:5 wait_for=<Future pending cb=[Task._wakeup()]>>

The loop stopped before the "create()" finished, "close()" has been
called before "write()", whereas coroutine functions were called in
this order: "create()", "write()", "close()".

To fix the example, tasks must be marked with "yield from":

   @asyncio.coroutine
   def test():
       yield from asyncio.async(create())
       yield from asyncio.async(write())
       yield from asyncio.async(close())
       yield from asyncio.sleep(2.0)
       loop.stop()

Or without "asyncio.async()":

   @asyncio.coroutine
   def test():
       yield from create()
       yield from write()
       yield from close()
       yield from asyncio.sleep(2.0)
       loop.stop()


Pending task destroyed
======================

If a pending task is destroyed, the execution of its wrapped
*coroutine* did not complete. It is probably a bug and so a warning is
logged.

Example of log:

   Task was destroyed but it is pending!
   source_traceback: Object created at (most recent call last):
     File "test.py", line 17, in <module>
       task = asyncio.async(coro, loop=loop)
     File "asyncio/tasks.py", line 510, in async
       task = loop.create_task(coro_or_future)
   task: <Task pending kill_me() done at test.py:5 wait_for=<Future pending cb=[Task._wakeup()]>>

*Enable the debug mode of asyncio* to get the traceback where the task
was created.

See also: *Detect coroutine objects never scheduled*.
