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
environment variable to 1.
Using the
.
Passing debug=True to
.
Calling
.
In addition to enabling the debug mode, consider also:
setting the log level of the
to
, for example the following snippet of code can be run at startup of the application:
logging.basicConfig(level=logging.DEBUG)
configuring the
module to display
warnings. One way of doing that is by using the
default command line option.
When the debug mode is enabled:
Many non-threadsafe asyncio APIs (such as
and
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
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
from another OS thread, the
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
method should be used, e.g.:
loop.call_soon_threadsafe(fut.cancel)To schedule a coroutine object from a different OS thread, the
function should be used. It returns a
to access the result:
asyncdefcoro_func():returnawaitasyncio.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
method can be used with a
concurrent.futures.ThreadPoolExecutor
or
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
). The
section lists APIs that can read from pipes and watch file descriptors without blocking the event loop. In addition, asyncio’s
APIs provide a way to start a process and communicate with it from the event loop. Lastly, the aforementioned
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
method for more details.
Logging
asyncio uses the
module and all logging is performed via the "asyncio" logger.
The default log level is
, 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 awaitcoro()) or the coroutine is not scheduled with
, asyncio will emit a
:
importasyncioasyncdeftest():print("never scheduled")asyncdefmain():test()asyncio.run(main())Output:
test.py:7:RuntimeWarning:coroutine'test'wasneverawaitedtest()Output in debug mode:
test.py:7:RuntimeWarning:coroutine'test'wasneverawaitedCoroutinecreatedat(mostrecentcalllast)File"../t.py",line9,in<module>asyncio.run(main(),debug=True)<..>File"../t.py",line7,inmaintest()test()The usual fix is to either await the coroutine or call the
function:
asyncdefmain():awaittest()Detect never-retrieved exceptions
If a
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:
importasyncioasyncdefbug():raiseException("not consumed")asyncdefmain():asyncio.create_task(bug())asyncio.run(main())Output:
Taskexceptionwasneverretrievedfuture:<Taskfinishedcoro=<bug()done,definedattest.py:3>exception=Exception('not consumed')>Traceback(mostrecentcalllast):File"test.py",line4,inbugraiseException("not consumed")Exception:notconsumed
to get the traceback where the task was created:
asyncio.run(main(),debug=True)Output in debug mode:
Taskexceptionwasneverretrievedfuture:<Taskfinishedcoro=<bug()done,definedattest.py:3>exception=Exception('not consumed')createdatasyncio/tasks.py:321>source_traceback:Objectcreatedat(mostrecentcalllast):File"../t.py",line9,in<module>asyncio.run(main(),debug=True)<..>Traceback(mostrecentcalllast):File"../t.py",line4,inbugraiseException("not consumed")Exception:notconsumedAsynchronous 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
. If a generator exits early - for example, due to an exception raised in the body of an asyncfor 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
method, or use the
context manager:
importasyncioimportcontextlibasyncdefgen():yield1yield2asyncdeffunc():asyncwithcontextlib.aclosing(gen())asg:asyncforxing:break# Don't iterate until the endasyncio.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:
importasynciowork_done=Falseasyncdefcursor():try:yield1finally:assertwork_doneasyncdefrows():globalwork_donetry:yield2finally:awaitasyncio.sleep(0.1)# imitate some async workwork_done=Trueasyncdefmain():asyncforcincursor():asyncforrinrows():breakbreakasyncio.run(main())For this example, we get the following output:
unhandledexceptionduringasyncio.run()shutdowntask:<Taskfinishedname='Task-3'coro=<<async_generator_athrowwithout__name__>()>exception=AssertionError()>Traceback(mostrecentcalllast):File"example.py",line6,incursoryield1asyncio.exceptions.CancelledErrorDuringhandlingoftheaboveexception,anotherexceptionoccurred:Traceback(mostrecentcalllast):File"example.py",line8,incursorassertwork_done^^^^^^^^^AssertionErrorThe 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:
asyncdefmain():asyncwithcontextlib.aclosing(cursor())ascursor_gen:asyncforcincursor_gen:asyncwithcontextlib.aclosing(rows())asrows_gen:asyncforrinrows_gen:breakbreakCreate asynchronous generators only when the event loop is running
It is recommended to create
only after the event loop has been created.
To ensure that asynchronous generators close reliably, the event loop uses the
function to register callback functions. These callbacks update the list of running asynchronous generators to keep it in a consistent state.
When the
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:
importasyncioasyncdefagenfn():try:yield10finally:awaitasyncio.sleep(0)withasyncio.Runner()asrunner:agen=agenfn()print(runner.run(anext(agen)))delagenOutput:
10Exceptionignoredwhileclosinggenerator<async_generatorobjectagenfnat0x000002F71CD10D70>:Traceback(mostrecentcalllast):File"example.py",line13,in<module>delagen^^^^RuntimeError:asyncgeneratorignoredGeneratorExitThis example can be fixed as follows:
importasyncioasyncdefagenfn():try:yield10finally:awaitasyncio.sleep(0)asyncdefmain():agen=agenfn()print(awaitanext(agen))delagenasyncio.run(main())Avoid concurrent iteration and closure of the same generator
Async generators may be reentered while another
/
/
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:
importasyncioasyncdefconsumer():foridxinrange(100):awaitasyncio.sleep(0)message=yieldidxprint('received',message)asyncdefamain():agenerator=consumer()awaitagenerator.asend(None)fa=asyncio.create_task(agenerator.asend('A'))fb=asyncio.create_task(agenerator.asend('B'))awaitfaawaitfbasyncio.run(amain())Output:
receivedATraceback(mostrecentcalllast):File"test.py",line38,in<module>asyncio.run(amain())~~~~~~~~~~~^^^^^^^^^File"Lib/asyncio/runners.py",line204,inrunreturnrunner.run(main)~~~~~~~~~~^^^^^^File"Lib/asyncio/runners.py",line127,inrunreturnself._loop.run_until_complete(task)~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^File"Lib/asyncio/base_events.py",line719,inrun_until_completereturnfuture.result()~~~~~~~~~~~~~^^File"test.py",line36,inamainawaitfbRuntimeError:anext():asynchronousgeneratorisalreadyrunningTherefore, it is recommended to avoid using asynchronous generators in parallel tasks or across multiple event loops.