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:

* 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 100 milliseconds 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 the event loop must be run in the main thread.

The "loop.run_in_executor()" method can be used with a
"concurrent.futures.ThreadPoolExecutor" or "InterpreterPoolExecutor"
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, including
in a different interpreter, 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)

Network logging can block the event loop. It is recommended to use a
separate thread for handling logs or use non-blocking IO. For example,
see Dealing with handlers that block.


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


Asynchronous generators best practices
======================================

Writing correct and efficient asyncio code requires awareness of
certain pitfalls. This section outlines essential best practices that
can save you hours of debugging.


Close asynchronous generators explicitly
----------------------------------------

It is recommended to manually close the *asynchronous generator*. If a
generator exits early - for example, due to an exception raised in the
body of an "async for" loop - its asynchronous cleanup code may run in
an unexpected context. This can occur after the tasks it depends on
have completed, or during the event loop shutdown when the async-
generator's garbage collection hook is called.

To avoid this, explicitly close the generator by calling its
"aclose()" method, or use the "contextlib.aclosing()" context manager:

   import asyncio
   import contextlib

   async def gen():
     yield 1
     yield 2

   async def func():
     async with contextlib.aclosing(gen()) as g:
       async for x in g:
         break  # Don't iterate until the end

   asyncio.run(func())

As noted above, the cleanup code for these asynchronous generators is
deferred. The following example demonstrates that the finalization of
an asynchronous generator can occur in an unexpected order:

   import asyncio
   work_done = False

   async def cursor():
       try:
           yield 1
       finally:
           assert work_done

   async def rows():
       global work_done
       try:
           yield 2
       finally:
           await asyncio.sleep(0.1) # immitate some async work
           work_done = True


   async def main():
       async for c in cursor():
           async for r in rows():
               break
           break

   asyncio.run(main())

For this example, we get the following output:

   unhandled exception during asyncio.run() shutdown
   task: <Task finished name='Task-3' coro=<<async_generator_athrow without __name__>()> exception=AssertionError()>
   Traceback (most recent call last):
     File "example.py", line 6, in cursor
       yield 1
   asyncio.exceptions.CancelledError

   During handling of the above exception, another exception occurred:

   Traceback (most recent call last):
     File "example.py", line 8, in cursor
       assert work_done
              ^^^^^^^^^
   AssertionError

The "cursor()" asynchronous generator was finalized before the "rows"
generator - an unexpected behavior.

The example can be fixed by explicitly closing the "cursor" and "rows"
async-generators:

   async def main():
       async with contextlib.aclosing(cursor()) as cursor_gen:
           async for c in cursor_gen:
               async with contextlib.aclosing(rows()) as rows_gen:
                   async for r in rows_gen:
                       break
               break


Create asynchronous generators only when the event loop is running
------------------------------------------------------------------

It is recommended to create *asynchronous generators* only after the
event loop has been created.

To ensure that asynchronous generators close reliably, the event loop
uses the "sys.set_asyncgen_hooks()" function to register callback
functions. These callbacks update the list of running asynchronous
generators to keep it in a consistent state.

When the "loop.shutdown_asyncgens()" function is called, the running
generators are stopped gracefully and the list is cleared.

The asynchronous generator invokes the corresponding system hook
during its first iteration. At the same time, the generator records
that the hook has been called and does not call it again.

Therefore, if iteration begins before the event loop is created, the
event loop will not be able to add the generator to its list of active
generators because the hooks are set after the generator attempts to
call them. Consequently, the event loop will not be able to terminate
the generator if necessary.

Consider the following example:

   import asyncio

   async def agenfn():
       try:
           yield 10
       finally:
           await asyncio.sleep(0)


   with asyncio.Runner() as runner:
       agen = agenfn()
       print(runner.run(anext(agen)))
       del agen

Output:

   10
   Exception ignored while closing generator <async_generator object agenfn at 0x000002F71CD10D70>:
   Traceback (most recent call last):
     File "example.py", line 13, in <module>
       del agen
           ^^^^
   RuntimeError: async generator ignored GeneratorExit

This example can be fixed as follows:

   import asyncio

   async def agenfn():
       try:
           yield 10
       finally:
           await asyncio.sleep(0)

   async def main():
       agen = agenfn()
       print(await anext(agen))
       del agen

   asyncio.run(main())


Avoid concurrent iteration and closure of the same generator
------------------------------------------------------------

Async generators may be reentered while another "__anext__()" /
"athrow()" / "aclose()" call is in progress. This may lead to an
inconsistent state of the async generator and can cause errors.

Let's consider the following example:

   import asyncio

   async def consumer():
       for idx in range(100):
           await asyncio.sleep(0)
           message = yield idx
           print('received', message)

   async def amain():
       agenerator = consumer()
       await agenerator.asend(None)

       fa = asyncio.create_task(agenerator.asend('A'))
       fb = asyncio.create_task(agenerator.asend('B'))
       await fa
       await fb

   asyncio.run(amain())

Output:

   received A
   Traceback (most recent call last):
     File "test.py", line 38, in <module>
       asyncio.run(amain())
       ~~~~~~~~~~~^^^^^^^^^
     File "Lib/asyncio/runners.py", line 204, in run
       return runner.run(main)
              ~~~~~~~~~~^^^^^^
     File "Lib/asyncio/runners.py", line 127, in run
       return self._loop.run_until_complete(task)
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
     File "Lib/asyncio/base_events.py", line 719, in run_until_complete
       return future.result()
              ~~~~~~~~~~~~~^^
     File "test.py", line 36, in amain
       await fb
   RuntimeError: anext(): asynchronous generator is already running

Therefore, it is recommended to avoid using asynchronous generators in
parallel tasks or across multiple event loops.
