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.. currentmodule:: asyncio
.. _asyncio-policies:
========
Policies
========
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.
Getting and Setting the Policy
==============================
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.
Policy Objects
==============
The abstract event loop policy base class is defined as follows:
.. class:: AbstractEventLoopPolicy
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)
Get the current child process watcher to *watcher*.
This function is Unix specific.
asyncio ships with the following built-in policies:
.. class:: DefaultEventLoopPolicy
The default asyncio policy. Uses :class:`SelectorEventLoop`
on both Unix and Windows platforms.
There is no need to install the default policy manually. asyncio
is configured to use the default policy automatically.
.. class:: WindowsProactorEventLoopPolicy
An alternative event loop policy that uses the
:class:`ProactorEventLoop` event loop implementation.
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
: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 two different
implementations: :class:`SafeChildWatcher` (configured to be used
by default) 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.
.. class:: AbstractChildWatcher
.. 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:: close()
Close the watcher.
This method has to be called to ensure that underlying
resources are cleaned-up.
.. class:: SafeChildWatcher
This implementation avoids disrupting other code spawning processes
by polling every process explicitly on a :py:data:`SIGCHLD` signal.
This is a safe solution but it has a significant overhead when
handling a big number of processes (*O(n)* each time a
:py:data:`SIGCHLD` is received).
asyncio uses this safe implementation by default.
.. class:: 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).
Custom Policies
===============
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())
|
