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

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


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 but not passed to "async()" or to
the "Task" constructor, it is not scheduled and it is probably a bug.

To detect such bug, set the environment variable "PYTHONASYNCIODEBUG"
to "1". When the coroutine object is destroyed by the garbage
collector, a log will be emitted with the traceback where the
coroutine function was called. See the *asyncio logger*.

The debug flag changes the behaviour of the "coroutine()" decorator.
The debug flag value is only used when then coroutine function is
defined, not when it is called.  Coroutine functions defined before
the debug flag is set to "True" will not be tracked. For example, it
is not possible to debug coroutines defined in the "asyncio" module,
because the module must be imported before the flag value can be
changed.

Example with the bug:

   import asyncio

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

   test()

Output in debug mode:

   Coroutine 'test' defined at test.py:4 was never yielded from

The fix is to call the "async()" function or create a "Task" object
with this coroutine object.


Detect exceptions not consumed
==============================

Python usually calls "sys.displayhook()" on unhandled exceptions. If
"Future.set_exception()" is called, but the exception is not 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. See the *asyncio logger*.

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:

   Future/Task exception was never retrieved:
   Traceback (most recent call last):
     File "/usr/lib/python3.4/asyncio/tasks.py", line 279, in _step
       result = next(coro)
     File "/usr/lib/python3.4/asyncio/tasks.py", line 80, 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 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 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.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(<create>)<PENDING>}

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()
