політики

An event loop policy is a global object used to get and set the current event loop, as well as create new event loops. The default policy can be replaced with built-in alternatives to use different event loop implementations, or substituted by a custom policy that can override these behaviors.

The policy object gets and sets a separate event loop per context. This is per-thread by default, though custom policies could define context differently.

Custom event loop policies can control the behavior of get_event_loop(), set_event_loop(), and new_event_loop().

Об’єкти політики мають реалізовувати API, визначені в AbstractEventLoopPolicy абстрактному базовому класі.

Отримання та налаштування політики

Наступні функції можна використовувати для отримання та налаштування політики для поточного процесу:

asyncio.get_event_loop_policy()

Повернути поточну політику для всього процесу.

asyncio.set_event_loop_policy(policy)

Установіть поточну політику для всього процесу на policy.

Якщо policy встановлено на None, політика за замовчуванням буде відновлена.

Об'єкти політики

Базовий клас політики абстрактного циклу подій визначається наступним чином:

class asyncio.AbstractEventLoopPolicy

Абстрактний базовий клас для асинхронних політик.

get_event_loop()

Отримати цикл подій для поточного контексту.

Повертає об’єкт циклу подій, що реалізує інтерфейс AbstractEventLoop.

Цей метод ніколи не повинен повертати None.

Berubah pada versi 3.6.

set_event_loop(loop)

Встановіть для циклу подій для поточного контексту значення loop.

new_event_loop()

Створити та повернути новий об’єкт циклу подій.

Цей метод ніколи не повинен повертати None.

get_child_watcher()

Get a child process watcher object.

Return a watcher object implementing the AbstractChildWatcher interface.

This function is Unix specific.

Ditinggalkan sejak versi 3.12.

set_child_watcher(watcher)

Set the current child process watcher to watcher.

This function is Unix specific.

Ditinggalkan sejak versi 3.12.

asyncio поставляється з такими вбудованими політиками:

class asyncio.DefaultEventLoopPolicy

Стандартна асинхронна політика. Використовує SelectorEventLoop в Unix і ProactorEventLoop у Windows.

Немає необхідності встановлювати стандартну політику вручну. asyncio налаштовано на автоматичне використання політики за замовчуванням.

Berubah pada versi 3.8: У Windows ProactorEventLoop тепер використовується за замовчуванням.

Ditinggalkan sejak versi 3.12: The get_event_loop() method of the default asyncio policy now emits a DeprecationWarning if there is no current event loop set and it decides to create one. In some future Python release this will become an error.

class asyncio.WindowsSelectorEventLoopPolicy

Альтернативна політика циклу подій, яка використовує реалізацію циклу подій SelectorEventLoop.

Availability: Windows.

class asyncio.WindowsProactorEventLoopPolicy

Альтернативна політика циклу подій, яка використовує реалізацію циклу подій ProactorEventLoop.

Availability: Windows.

Process Watchers

A process watcher allows customization of how an event loop monitors child processes on Unix. Specifically, the event loop needs to know when a child process has exited.

In asyncio, child processes are created with create_subprocess_exec() and loop.subprocess_exec() functions.

asyncio defines the AbstractChildWatcher abstract base class, which child watchers should implement, and has four different implementations: ThreadedChildWatcher (configured to be used by default), MultiLoopChildWatcher, SafeChildWatcher, and FastChildWatcher.

See also the Subprocess and Threads section.

The following two functions can be used to customize the child process watcher implementation used by the asyncio event loop:

asyncio.get_child_watcher()

Return the current child watcher for the current policy.

Ditinggalkan sejak versi 3.12.

asyncio.set_child_watcher(watcher)

Set the current child watcher to watcher for the current policy. watcher must implement methods defined in the AbstractChildWatcher base class.

Ditinggalkan sejak versi 3.12.

Catatan

Third-party event loops implementations might not support custom child watchers. For such event loops, using set_child_watcher() might be prohibited or have no effect.

class asyncio.AbstractChildWatcher

add_child_handler(pid, callback, *args)

Register a new child handler.

Arrange for callback(pid, returncode, *args) to be called when a process with PID equal to pid terminates. Specifying another callback for the same process replaces the previous handler.

The callback callable must be thread-safe.

remove_child_handler(pid)

Removes the handler for process with PID equal to pid.

The function returns True if the handler was successfully removed, False if there was nothing to remove.

attach_loop(loop)

Attach the watcher to an event loop.

If the watcher was previously attached to an event loop, then it is first detached before attaching to the new loop.

Note: loop may be None.

is_active()

Return True if the watcher is ready to use.

Spawning a subprocess with inactive current child watcher raises RuntimeError.

Added in version 3.8.

close()

Close the watcher.

This method has to be called to ensure that underlying resources are cleaned-up.

Ditinggalkan sejak versi 3.12.

class asyncio.ThreadedChildWatcher

This implementation starts a new waiting thread for every subprocess spawn.

It works reliably even when the asyncio event loop is run in a non-main OS thread.

There is no noticeable overhead when handling a big number of children (O(1) each time a child terminates), but starting a thread per process requires extra memory.

This watcher is used by default.

Added in version 3.8.

class asyncio.MultiLoopChildWatcher

This implementation registers a SIGCHLD signal handler on instantiation. That can break third-party code that installs a custom handler for SIGCHLD signal.

The watcher avoids disrupting other code spawning processes by polling every process explicitly on a SIGCHLD signal.

There is no limitation for running subprocesses from different threads once the watcher is installed.

The solution is safe but it has a significant overhead when handling a big number of processes (O(n) each time a SIGCHLD is received).

Added in version 3.8.

Ditinggalkan sejak versi 3.12.

class asyncio.SafeChildWatcher

This implementation uses active event loop from the main thread to handle SIGCHLD signal. If the main thread has no running event loop another thread cannot spawn a subprocess (RuntimeError is raised).

The watcher avoids disrupting other code spawning processes by polling every process explicitly on a SIGCHLD signal.

This solution is as safe as MultiLoopChildWatcher and has the same O(n) complexity but requires a running event loop in the main thread to work.

Ditinggalkan sejak versi 3.12.

class asyncio.FastChildWatcher

This implementation reaps every terminated processes by calling os.waitpid(-1) directly, possibly breaking other code spawning processes and waiting for their termination.

There is no noticeable overhead when handling a big number of children (O(1) each time a child terminates).

This solution requires a running event loop in the main thread to work, as SafeChildWatcher.

Ditinggalkan sejak versi 3.12.

class asyncio.PidfdChildWatcher

This implementation polls process file descriptors (pidfds) to await child process termination. In some respects, PidfdChildWatcher is a "Goldilocks" child watcher implementation. It doesn't require signals or threads, doesn't interfere with any processes launched outside the event loop, and scales linearly with the number of subprocesses launched by the event loop. The main disadvantage is that pidfds are specific to Linux, and only work on recent (5.3+) kernels.

Added in version 3.9.

Спеціальна політика

Щоб запровадити нову політику циклу подій, рекомендується створити підклас DefaultEventLoopPolicy і перевизначити методи, для яких потрібна спеціальна поведінка, наприклад:

class MyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):

    def get_event_loop(self):
        """Get the event loop.

        This may be None or an instance of EventLoop.
        """
        loop = super().get_event_loop()
        # Do something with loop ...
        return loop

asyncio.set_event_loop_policy(MyEventLoopPolicy())