ポリシー
********

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()".

ポリシーオブジェクトは "AbstractEventLoopPolicy" 抽象基底クラスで定義
された API を実装しなければなりません。


ポリシーの取得と設定
====================

以下の関数は現在のプロセスに対するポリシーの取得や設定をするために使わ
れます:

asyncio.get_event_loop_policy()

   プロセス全体にわたる現在のポリシーを返します。

asyncio.set_event_loop_policy(policy)

   プロセス全体にわたる現在のポリシーを *policy* に設定します。

   *policy* が "None" の場合、デフォルトポリシーが現在のポリシーに戻さ
   れます。


ポリシーオブジェクト
====================

イベントループポリシーの抽象基底クラスは以下のように定義されています:

class asyncio.AbstractEventLoopPolicy

   asyncio ポリシーの抽象基底クラスです。

   get_event_loop()

      現在のコンテキストのイベントループを取得します。

      "AbstractEventLoop" のインターフェースを実装したイベントループオ
      ブジェクトを返します。

      このメソッドは "None" を返してはいけません。

      バージョン 3.6 で変更.

   set_event_loop(loop)

      現在のコンテキストにイベントループ *loop* を設定します。

   new_event_loop()

      新しいイベントループオブジェクトを生成して返します。

      このメソッドは "None" を返してはいけません。

   get_child_watcher()

      Get a child process watcher object.

      Return a watcher object implementing the "AbstractChildWatcher"
      interface.

      This function is Unix specific.

      バージョン 3.12 で非推奨.

   set_child_watcher(watcher)

      Set the current child process watcher to *watcher*.

      This function is Unix specific.

      バージョン 3.12 で非推奨.

asyncio は以下の組み込みポリシーを提供します:

class asyncio.DefaultEventLoopPolicy

   デフォルトの asyncio ポリシーです。Unix では "SelectorEventLoop" 、
   Windows では "ProactorEventLoop" を使います。

   デフォルトのポリシーを手動でインストールする必要はありません。
   asyncio はデフォルトポリシーを使うように自動的に構成されます。

   バージョン 3.8 で変更: Windows では "ProactorEventLoop" がデフォル
   トで使われるようになりました。

   バージョン 3.12 で非推奨: 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

   "SelectorEventLoop" イベントループ実装を使った別のイベントループポ
   リシーです。

   Availability: Windows.

class asyncio.WindowsProactorEventLoopPolicy

   "ProactorEventLoop" イベントループ実装を使った別のイベントループポ
   リシーです。

   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.

   バージョン 3.12 で非推奨.

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.

   バージョン 3.12 で非推奨.

注釈:

  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.

   バージョン 3.12 で非推奨.

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.

   バージョン 3.12 で非推奨.

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.

   バージョン 3.12 で非推奨.

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".

   バージョン 3.12 で非推奨.

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.


ポリシーのカスタマイズ
======================

新しいイベントループのポリシーを実装するためには、以下に示すように
"DefaultEventLoopPolicy" を継承して振る舞いを変更したいメソッドをオー
バーライドすることが推奨されます。:

   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())
