18.5.9. Develop with asyncio
****************************

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


18.5.9.1. Debug mode of asyncio
===============================

The implementation of "asyncio" has been written for performance. In
order to ease the development of asynchronous code, you may wish to
enable *debug mode*.

To enable all debug checks for an application:

* Enable the asyncio debug mode globally by setting the environment
  variable "PYTHONASYNCIODEBUG" to "1", or by calling
  "BaseEventLoop.set_debug()".

* Set the log level of the asyncio logger to "logging.DEBUG". For
  example, call "logging.basicConfig(level=logging.DEBUG)" at startup.

* Configure the "warnings" module to display "ResourceWarning"
  warnings. For example, use the "-Wdefault" command line option of
  Python to display them.

Examples debug checks:

* 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.

* "ResourceWarning" warnings are emitted when transports and event
  loops are not closed explicitly.

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


18.5.9.2. Cancellation
======================

Cancellation of tasks is not common in classic programming. In
asynchronous programming, not only it is something common, but you
have to prepare your code to handle it.

Futures and tasks can be cancelled explicitly with their
"Future.cancel()" method. The "wait_for()" function cancels the waited
task when the timeout occurs. There are many other cases where a task
can be cancelled indirectly.

Don’t call "set_result()" or "set_exception()" method of "Future" if
the future is cancelled: it would fail with an exception. For example,
write:

   if not fut.cancelled():
       fut.set_result('done')

Don’t schedule directly a call to the "set_result()" or the
"set_exception()" method of a future with "BaseEventLoop.call_soon()":
the future can be cancelled before its method is called.

If you wait for a future, you should check early if the future was
cancelled to avoid useless operations. Example:

   @coroutine
   def slow_operation(fut):
       if fut.cancelled():
           return
       # ... slow computation ...
       yield from fut
       # ...

The "shield()" function can also be used to ignore cancellation.


18.5.9.3. 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:

   loop.call_soon_threadsafe(callback, *args)

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 schedule a coroutine object from a different thread, the
"run_coroutine_threadsafe()" function should be used. It returns a
"concurrent.futures.Future" to access the result:

   future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
   result = future.result(timeout)  # Wait for the result with a timeout

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: The Synchronization primitives section describes ways to
  synchronize tasks.

  The Subprocess and threads section lists asyncio limitations to run
  subprocesses from different threads.


18.5.9.4. 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.


18.5.9.5. Logging
=================

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


18.5.9.6. 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 object will never be scheduled which 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.


18.5.9.7. 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 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.ensure_future(bug())
   loop.run_forever()
   loop.close()

Output:

   Task exception was never retrieved
   future: <Task finished coro=<coro() done, defined at asyncio/coroutines.py:139> exception=Exception('not consumed',)>
   Traceback (most recent call last):
     File "asyncio/tasks.py", line 237, in _step
       result = next(coro)
     File "asyncio/coroutines.py", line 141, in coro
       res = func(*args, **kw)
     File "test.py", line 5, in bug
       raise Exception("not consumed")
   Exception: not consumed

Enable the debug mode of asyncio to get the traceback where the task
was created. Output in debug mode:

   Task exception was never retrieved
   future: <Task finished coro=<bug() done, defined at test.py:3> exception=Exception('not consumed',) created at test.py:8>
   source_traceback: Object created at (most recent call last):
     File "test.py", line 8, in <module>
       asyncio.ensure_future(bug())
   Traceback (most recent call last):
     File "asyncio/tasks.py", line 237, in _step
       result = next(coro)
     File "asyncio/coroutines.py", line 79, in __next__
       return next(self.gen)
     File "asyncio/coroutines.py", line 141, in coro
       res = func(*args, **kw)
     File "test.py", line 5, in bug
       raise Exception("not consumed")
   Exception: not consumed

There are different options to fix this issue. The first option is to
chain the 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.ensure_future(handle_exception())
   loop.run_forever()
   loop.close()

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

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

See also: The "Future.exception()" method.


18.5.9.8. Chain coroutines correctly
====================================

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.ensure_future(create())
       asyncio.ensure_future(write())
       asyncio.ensure_future(close())
       yield from asyncio.sleep(2.0)
       loop.stop()

   loop = asyncio.get_event_loop()
   asyncio.ensure_future(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.ensure_future(create())
       yield from asyncio.ensure_future(write())
       yield from asyncio.ensure_future(close())
       yield from asyncio.sleep(2.0)
       loop.stop()

Or without "asyncio.ensure_future()":

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


18.5.9.9. 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!
   task: <Task pending coro=<kill_me() done, defined 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. Example of log in debug mode:

   Task was destroyed but it is pending!
   source_traceback: Object created at (most recent call last):
     File "test.py", line 15, in <module>
       task = asyncio.ensure_future(coro, loop=loop)
   task: <Task pending coro=<kill_me() done, defined at test.py:5> wait_for=<Future pending cb=[Task._wakeup()] created at test.py:7> created at test.py:15>

See also: Detect coroutine objects never scheduled.


18.5.9.10. Close transports and event loops
===========================================

When a transport is no more needed, call its "close()" method to
release resources. Event loops must also be closed explicitly.

If a transport or an event loop is not closed explicitly, a
"ResourceWarning" warning will be emitted in its destructor. By
default, "ResourceWarning" warnings are ignored. The Debug mode of
asyncio section explains how to display them.
