.. currentmodule:: asyncio .. _asyncio-dev: ======================= Developing with asyncio ======================= Asynchronous programming is different from classic "sequential" programming. This page lists common mistakes and traps and explains how to avoid them. .. _asyncio-debug-mode: 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 :envvar:`PYTHONASYNCIODEBUG` environment variable to ``1``. * Using the :ref:`Python Development Mode `. * Passing ``debug=True`` to :func:`asyncio.run`. * Calling :meth:`loop.set_debug`. In addition to enabling the debug mode, consider also: * setting the log level of the :ref:`asyncio logger ` to :py:const:`logging.DEBUG`, for example the following snippet of code can be run at startup of the application:: logging.basicConfig(level=logging.DEBUG) * configuring the :mod:`warnings` module to display :exc:`ResourceWarning` warnings. One way of doing that is by using the :option:`-W` ``default`` command line option. When the debug mode is enabled: * Many non-threadsafe asyncio APIs (such as :meth:`loop.call_soon` and :meth:`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 :attr:`loop.slow_callback_duration` attribute can be used to set the minimum execution duration in seconds that is considered "slow". .. _asyncio-multithreading: 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 :term:`callback` from another OS thread, the :meth:`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 :meth:`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 :func:`run_coroutine_threadsafe` function should be used. It returns a :class:`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 :meth:`loop.run_in_executor` method can be used with a :class:`concurrent.futures.ThreadPoolExecutor` or :class:`~concurrent.futures.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 :mod:`multiprocessing`). The :ref:`asyncio-event-loop-methods` section lists APIs that can read from pipes and watch file descriptors without blocking the event loop. In addition, asyncio's :ref:`Subprocess ` APIs provide a way to start a process and communicate with it from the event loop. Lastly, the aforementioned :meth:`loop.run_in_executor` method can also be used with a :class:`concurrent.futures.ProcessPoolExecutor` to execute code in a different process. .. _asyncio-handle-blocking: 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 :meth:`loop.run_in_executor` method for more details. .. _asyncio-logger: Logging ======= asyncio uses the :mod:`logging` module and all logging is performed via the ``"asyncio"`` logger. The default log level is :py:const:`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 :ref:`blocking-handlers`. .. _asyncio-coroutine-not-scheduled: 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 :meth:`asyncio.create_task`, asyncio will emit a :exc:`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 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 :meth:`asyncio.create_task` function:: async def main(): await test() Detect never-retrieved exceptions ================================= If a :meth:`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: exception=Exception('not consumed')> Traceback (most recent call last): File "test.py", line 4, in bug raise Exception("not consumed") Exception: not consumed :ref:`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: 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 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 :term:`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 :meth:`~agen.aclose` method, or use the :func:`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: ()> 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 :term:`asynchronous generators ` only after the event loop has been created. To ensure that asynchronous generators close reliably, the event loop uses the :func:`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 :meth:`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 : Traceback (most recent call last): File "example.py", line 13, in 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 :meth:`~agen.__anext__` / :meth:`~agen.athrow` / :meth:`~agen.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 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.