Developing with asyncio
***********************

Asynchronous programming is different from classic “sequential”
programming.

This page lists common mistakes and traps and explains how to avoid
them.


Debug Mode
==========

By default asyncio runs in production mode.  In order to ease the
development asyncio has a *debug mode*.

There are several ways to enable asyncio debug mode:

* Setting the "PYTHONASYNCIODEBUG" environment variable to "1".

* Using the Python Development Mode.

* Passing "debug=True" to "asyncio.run()".

* Calling "loop.set_debug()".

In addition to enabling the debug mode, consider also:

* setting the log level of the asyncio logger to "logging.DEBUG", for
  example the following snippet of code can be run at startup of the
  application:

     logging.basicConfig(level=logging.DEBUG)

* configuring the "warnings" module to display "ResourceWarning"
  warnings.  One way of doing that is by using the "-W" "default"
  command line option.

When the debug mode is enabled:

* asyncio checks for coroutines that were not awaited and logs them;
  this mitigates the “forgotten await” pitfall.

* Many non-threadsafe asyncio APIs (such as "loop.call_soon()" and
  "loop.call_at()" methods) raise an exception if they are called from
  a wrong thread.

* The execution time of the I/O selector is logged if it takes too
  long to perform an I/O operation.

* Callbacks taking longer than 100ms are logged.  The
  "loop.slow_callback_duration" attribute can be used to set the
  minimum execution duration in seconds that is considered “slow”.


Concurrency and Multithreading
==============================

An event loop runs in a thread (typically the main thread) and
executes all callbacks and Tasks in its thread.  While a Task is
running in the event loop, no other Tasks can run in the same thread.
When a Task executes an "await" expression, the running Task gets
suspended, and the event loop executes the next Task.

To schedule a *callback* from another OS thread, the
"loop.call_soon_threadsafe()" method should be used. Example:

   loop.call_soon_threadsafe(callback, *args)

Almost all asyncio objects are not thread safe, which is typically not
a problem unless there is code that works with them from outside of a
Task or a callback.  If there’s a need for such code to call a low-
level asyncio API, the "loop.call_soon_threadsafe()" method should be
used, e.g.:

   loop.call_soon_threadsafe(fut.cancel)

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

   async def coro_func():
        return await asyncio.sleep(1, 42)

   # Later in another OS thread:

   future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
   # Wait for the result:
   result = future.result()

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

The "loop.run_in_executor()" method can be used with a
"concurrent.futures.ThreadPoolExecutor" to execute blocking code in a
different OS thread without blocking the OS thread that the event loop
runs in.

There is currently no way to schedule coroutines or callbacks directly
from a different process (such as one started with "multiprocessing").
The Event Loop Methods section lists APIs that can read from pipes and
watch file descriptors without blocking the event loop. In addition,
asyncio’s Subprocess APIs provide a way to start a process and
communicate with it from the event loop. Lastly, the aforementioned
"loop.run_in_executor()" method can also be used with a
"concurrent.futures.ProcessPoolExecutor" to execute code in a
different process.


Running Blocking Code
=====================

Blocking (CPU-bound) code should not be called directly.  For example,
if a function performs a CPU-intensive calculation for 1 second, all
concurrent asyncio Tasks and IO operations would be delayed by 1
second.

An executor can be used to run a task in a different thread or even in
a different process to avoid blocking the OS thread with the event
loop.  See the "loop.run_in_executor()" method for more details.


Logging
=======

asyncio uses the "logging" module and all logging is performed via the
""asyncio"" logger.

The default log level is "logging.INFO", which can be easily adjusted:

   logging.getLogger("asyncio").setLevel(logging.WARNING)


Detect never-awaited coroutines
===============================

When a coroutine function is called, but not awaited (e.g. "coro()"
instead of "await coro()") or the coroutine is not scheduled with
"asyncio.create_task()", asyncio will emit a "RuntimeWarning":

   import asyncio

   async def test():
       print("never scheduled")

   async def main():
       test()

   asyncio.run(main())

Output:

   test.py:7: RuntimeWarning: coroutine 'test' was never awaited
     test()

Output in debug mode:

   test.py:7: RuntimeWarning: coroutine 'test' was never awaited
   Coroutine created at (most recent call last)
     File "../t.py", line 9, in <module>
       asyncio.run(main(), debug=True)

     < .. >

     File "../t.py", line 7, in main
       test()
     test()

The usual fix is to either await the coroutine or call the
"asyncio.create_task()" function:

   async def main():
       await test()


Detect never-retrieved exceptions
=================================

If a "Future.set_exception()" is called but the Future object is never
awaited on, the exception would never be propagated to the user code.
In this case, asyncio would emit a log message when the Future object
is garbage collected.

Example of an unhandled exception:

   import asyncio

   async def bug():
       raise Exception("not consumed")

   async def main():
       asyncio.create_task(bug())

   asyncio.run(main())

Output:

   Task exception was never retrieved
   future: <Task finished coro=<bug() done, defined at test.py:3>
     exception=Exception('not consumed')>

   Traceback (most recent call last):
     File "test.py", line 4, in bug
       raise Exception("not consumed")
   Exception: not consumed

Enable the debug mode to get the traceback where the task was created:

   asyncio.run(main(), debug=True)

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 asyncio/tasks.py:321>

   source_traceback: Object created at (most recent call last):
     File "../t.py", line 9, in <module>
       asyncio.run(main(), debug=True)

   < .. >

   Traceback (most recent call last):
     File "../t.py", line 4, in bug
       raise Exception("not consumed")
   Exception: not consumed
