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)
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 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 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)
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())
test.py:7: RuntimeWarning: coroutine 'test' was never awaited test()
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())
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:
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
Table of Contents
asyncio — Asynchronous I/O¶
asyncio is a library to write concurrent code using the async/await syntax.
asyncio is used as a foundation for multiple Python asynchronous frameworks that provide high-performance network and web-servers, database connection libraries, distributed task queues, etc.
asyncio is often a perfect fit for IO-bound and high-level structured network code.
asyncio provides a set of high-level APIs to:
- run Python coroutines concurrently and have full control over their execution;
- perform network IO and IPC ;
- control subprocesses ;
- distribute tasks via queues ;
- synchronize concurrent code;
Additionally, there are low-level APIs for library and framework developers to:
- create and manage event loops , which provide asynchronous APIs for networking , running subprocesses , handling OS signals , etc;
- implement efficient protocols using transports ;
- bridge callback-based libraries and code with async/await syntax.
You can experiment with an asyncio concurrent context in the REPL:
$ python -m asyncio asyncio REPL . Use "await" directly instead of "asyncio.run()". Type "help", "copyright", "credits" or "license" for more information. >>> import asyncio >>> await asyncio.sleep(10, result='hello') 'hello'
This module does not work or is not available on WebAssembly platforms wasm32-emscripten and wasm32-wasi . See WebAssembly platforms for more information.