.. currentmodule:: asyncio
An event loop policy is a global per-process object that controls the management of the event loop. Each event loop has a default policy, which can be changed and customized using the policy API.
A policy defines the notion of context and manages a separate event loop per context. The default policy defines context to be the current thread.
By using a custom event loop policy, the behavior of :func:`get_event_loop`, :func:`set_event_loop`, and :func:`new_event_loop` functions can be customized.
Policy objects should implement the APIs defined in the :class:`AbstractEventLoopPolicy` abstract base class.
The following functions can be used to get and set the policy for the current process:
.. function:: get_event_loop_policy() Return the current process-wide policy.
.. function:: set_event_loop_policy(policy) Set the current process-wide policy to *policy*. If *policy* is set to ``None``, the default policy is restored.
The abstract event loop policy base class is defined as follows:
An abstract base class for asyncio policies.
.. method:: get_event_loop() Get the event loop for the current context. Return an event loop object implementing the :class:`AbstractEventLoop` interface. This method should never return ``None``. .. versionchanged:: 3.6
.. method:: set_event_loop(loop) Set the event loop for the current context to *loop*.
.. method:: new_event_loop() Create and return a new event loop object. This method should never return ``None``.
.. method:: get_child_watcher() Get a child process watcher object. Return a watcher object implementing the :class:`AbstractChildWatcher` interface. This function is Unix specific.
.. method:: set_child_watcher(watcher) Set the current child process watcher to *watcher*. This function is Unix specific.
asyncio ships with the following built-in policies:
The default asyncio policy. Uses :class:`SelectorEventLoop` on Unix and :class:`ProactorEventLoop` on Windows.
There is no need to install the default policy manually. asyncio is configured to use the default policy automatically.
.. versionchanged:: 3.8 On Windows, :class:`ProactorEventLoop` is now used by default.
An alternative event loop policy that uses the :class:`SelectorEventLoop` event loop implementation.
.. availability:: Windows.
An alternative event loop policy that uses the :class:`ProactorEventLoop` event loop implementation.
.. availability:: Windows.
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 :func:`create_subprocess_exec` and :meth:`loop.subprocess_exec` functions.
asyncio defines the :class:`AbstractChildWatcher` abstract base class, which child watchers should implement, and has four different implementations: :class:`ThreadedChildWatcher` (configured to be used by default), :class:`MultiLoopChildWatcher`, :class:`SafeChildWatcher`, and :class:`FastChildWatcher`.
See also the :ref:`Subprocess and Threads <asyncio-subprocess-threads>` section.
The following two functions can be used to customize the child process watcher implementation used by the asyncio event loop:
.. function:: get_child_watcher() Return the current child watcher for the current policy.
.. function:: set_child_watcher(watcher) Set the current child watcher to *watcher* for the current policy. *watcher* must implement methods defined in the :class:`AbstractChildWatcher` base class.
Note
Third-party event loops implementations might not support custom child watchers. For such event loops, using :func:`set_child_watcher` might be prohibited or have no effect.
.. method:: 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.
.. method:: 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.
.. method:: 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``.
.. method:: is_active() Return ``True`` if the watcher is ready to use. Spawning a subprocess with *inactive* current child watcher raises :exc:`RuntimeError`. .. versionadded:: 3.8
.. method:: close() Close the watcher. This method has to be called to ensure that underlying resources are cleaned-up.
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 stating a thread per process requires extra memory.
This watcher is used by default.
.. versionadded:: 3.8
This implementation registers a :py:data:`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 :py:data:`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 :py:data:`SIGCHLD` is received).
.. versionadded:: 3.8
This implementation uses active event loop from the main thread to handle :py:data:`SIGCHLD` signal. If the main thread has no running event loop another thread cannot spawn a subprocess (:exc:`RuntimeError` is raised).
The watcher avoids disrupting other code spawning processes by polling every process explicitly on a :py:data:`SIGCHLD` signal.
This solution is as safe as :class:`MultiLoopChildWatcher` and has the same O(N) complexity but requires a running event loop in the main thread to work.
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 :class:`SafeChildWatcher`.
To implement a new event loop policy, it is recommended to subclass :class:`DefaultEventLoopPolicy` and override the methods for which custom behavior is wanted, e.g.:
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())
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