事件循环
********

**Source code:** Lib/asyncio/events.py, Lib/asyncio/base_events.py

======================================================================

-[ 前言 ]-

事件循环是每个 asyncio 应用的核心。 事件循环会运行异步任务和回调，执行
网络 IO 操作，以及运行子进程。

应用开发者通常应当使用高层级的 asyncio 函数，例如 "asyncio.run()"，应
当很少有必要引用循环对象或调用其方法。 本节所针对的主要是低层级代码、
库和框架的编写者，他们需要更细致地控制事件循环行为。

-[ 获取事件循环 ]-

以下低层级函数可被用于获取、设置或创建事件循环:

asyncio.get_running_loop()

   返回当前 OS 线程中正在运行的事件循环。

   如果没有正在运行的事件循环则会引发 "RuntimeError"。 此函数只能由协
   程或回调来调用。

   3.7 版新加入.

asyncio.get_event_loop()

   Get the current event loop.

   If there is no current event loop set in the current OS thread, the
   OS thread is main, and "set_event_loop()" has not yet been called,
   asyncio will create a new event loop and set it as the current one.

   由于此函数具有相当复杂的行为（特别是在使用了自定义事件循环策略的时
   候），更推荐在协程和回调中使用 "get_running_loop()" 函数而非
   "get_event_loop()"。

   应该考虑使用 "asyncio.run()" 函数而非使用低层级函数来手动创建和关闭
   事件循环。

asyncio.set_event_loop(loop)

   将 *loop* 设置为当前 OS 线程的当前事件循环。

asyncio.new_event_loop()

   创建一个新的事件循环。

请注意 "get_event_loop()", "set_event_loop()" 以及 "new_event_loop()"
函数的行为可以通过 设置自定义事件循环策略 来改变。

-[ 目录 ]-

本文档包含下列小节:

* 事件循环方法集 章节是事件循环APIs的参考文档；

* 回调处理 章节是从调度方法 例如 "loop.call_soon()"  和
  "loop.call_later()" 中返回 "Handle" 和 "TimerHandle" 实例的文档。

* Server Objects 章节记录了从事件循环方法返回的类型，比如
  "loop.create_server()" ；

* Event Loop Implementations 章节记录了 "SelectorEventLoop" 和
  "ProactorEventLoop" 类；

* Examples 章节展示了如何使用某些事件循环API。


事件循环方法集
==============

事件循环有下列 **低级** APIs：

* 运行和停止循环

* 调度回调

* 调度延迟回调

* 创建 Futures 和 Tasks

* 打开网络连接

* 创建网络服务

* 传输文件

* TLS 升级

* 监控文件描述符

* 直接使用 socket 对象

* DNS

* 使用管道

* Unix 信号

* 在线程或者进程池中执行代码。

* 错误处理API

* 开启调试模式

* 运行子进程


运行和停止循环
--------------

loop.run_until_complete(future)

   运行直到 *future* (  "Future" 的实例 ) 被完成。

   如果参数是 coroutine object ，将被隐式调度为 "asyncio.Task" 来运行
   。

   返回 Future 的结果 或者引发相关异常。

loop.run_forever()

   运行事件循环直到 "stop()" 被调用。

   If "stop()" is called before "run_forever()" is called, the loop
   will poll the I/O selector once with a timeout of zero, run all
   callbacks scheduled in response to I/O events (and those that were
   already scheduled), and then exit.

   If "stop()" is called while "run_forever()" is running, the loop
   will run the current batch of callbacks and then exit. Note that
   new callbacks scheduled by callbacks will not run in this case;
   instead, they will run the next time "run_forever()" or
   "run_until_complete()" is called.

loop.stop()

   停止事件循环。

loop.is_running()

   返回 "True" 如果事件循环当前正在运行。

loop.is_closed()

   如果事件循环已经被关闭，返回 "True" 。

loop.close()

   关闭事件循环。

   当这个函数被调用的时候，循环必须处于非运行状态。pending状态的回调将
   被丢弃。

   此方法清除所有的队列并立即关闭执行器，不会等待执行器完成。

   这个方法是幂等的和不可逆的。事件循环关闭后，不应调用其他方法。

coroutine loop.shutdown_asyncgens()

   Schedule all currently open *asynchronous generator* objects to
   close with an "aclose()" call.  After calling this method, the
   event loop will issue a warning if a new asynchronous generator is
   iterated. This should be used to reliably finalize all scheduled
   asynchronous generators.

   运行请注意，当使用 "asyncio.run()" 时，无需调用此函数。

   示例:

      try:
          loop.run_forever()
      finally:
          loop.run_until_complete(loop.shutdown_asyncgens())
          loop.close()

   3.6 版新加入.


调度回调
--------

loop.call_soon(callback, *args, context=None)

   Schedule the *callback* *callback* to be called with *args*
   arguments at the next iteration of the event loop.

   回调按其注册顺序被调用。每个回调仅被调用一次。

   可选键值类的参数 *context* 允许 *callback* 运行在一个指定的自定义
   "contextvars.Context" 对象中。如果没有提供 *context* ，则使用当前上
   下文。

   返回一个能用来取消回调的 "asyncio.Handle" 实例。

   这个方法不是线程安全的。

loop.call_soon_threadsafe(callback, *args, context=None)

   "call_soon()" 的线程安全变体。必须被用于安排 *来自其他线程* 的回调
   。

   参见 concurrency and multithreading 部分的文档。

3.7 版更變: 加入键值类形参  *context*。请参阅 **PEP 567** 查看更多细节
。

備註:

  大多数 "asyncio" 的调度函数不让传递关键字参数。为此，请使用
  "functools.partial()" ：

     # will schedule "print("Hello", flush=True)"
     loop.call_soon(
         functools.partial(print, "Hello", flush=True))

  使用 partial 对象通常比使用lambda更方便，asyncio 在调试和错误消息中
  能更好的呈现 partial 对象。


调度延迟回调
------------

事件循环提供安排调度函数在将来某个时刻调用的机制。事件循环使用单调时钟
来跟踪时间。

loop.call_later(delay, callback, *args, context=None)

   安排 *callback* 在给定的 *delay* 秒（可以是 int 或者 float）后被调
   用。

   返回一个 "asyncio.TimerHandle" 实例，该实例能用于取消回调。

   *callback* 只被调用一次。如果两个回调被安排在同样的时间点，执行顺序
   未限定。

   可选的位置参数 *args* 在被调用的时候传递给  *callback*  。 如果你想
   把关键字参数传递给 *callback* ，请使用 "functools.partial()" 。

   可选键值类的参数 *context* 允许 *callback* 运行在一个指定的自定义
   "contextvars.Context" 对象中。如果没有提供 *context* ，则使用当前上
   下文。

   3.7 版更變: 加入键值类形参  *context*。请参阅 **PEP 567** 查看更多
   细节。

   3.8 版更變: In Python 3.7 and earlier with the default event loop
   implementation, the *delay* could not exceed one day. This has been
   fixed in Python 3.8.

loop.call_at(when, callback, *args, context=None)

   安排 *callback* 在给定的绝对时间戳的 *时间* （一个 int 或者 float）
   被调用，使用与 "loop.time()" 同样的时间参考。

   本方法的行为和 "call_later()" 方法相同。

   返回一个 "asyncio.TimerHandle" 实例，该实例能用于取消回调。

   3.7 版更變: 加入键值类形参  *context*。请参阅 **PEP 567** 查看更多
   细节。

   3.8 版更變: In Python 3.7 and earlier with the default event loop
   implementation, the difference between *when* and the current time
   could not exceed one day.  This has been fixed in Python 3.8.

loop.time()

   根据时间循环内部的单调时钟，返回当前时间， "float" 值。

備註:

  3.8 版更變: In Python 3.7 and earlier timeouts (relative *delay* or
  absolute *when*) should not exceed one day.  This has been fixed in
  Python 3.8.

也參考: "asyncio.sleep()" 函数。


创建 Futures 和 Tasks
---------------------

loop.create_future()

   创建一个附加到事件循环中的 "asyncio.Future" 对象。

   这是在asyncio中创建Futures的首选方式。这让第三方事件循环可以提供
   Future 对象的替代实现(更好的性能或者功能)。

   3.5.2 版新加入.

loop.create_task(coro, *, name=None)

   安排一个 协程 的执行。返回一个  "Task"  对象。

   第三方的事件循环可以使用它们自己的 "Task" 子类来满足互操作性。这种
   情况下结果类型是一个 "Task" 的子类。

   If the *name* argument is provided and not "None", it is set as the
   name of the task using "Task.set_name()".

   3.8 版更變: 添加了 "name" 形参。

loop.set_task_factory(factory)

   设置一个 task 工厂 ， 被用于  "loop.create_task()" 。

   If *factory* is "None" the default task factory will be set.
   Otherwise, *factory* must be a *callable* with the signature
   matching "(loop, coro)", where *loop* is a reference to the active
   event loop, and *coro* is a coroutine object.  The callable must
   return a "asyncio.Future"-compatible object.

loop.get_task_factory()

   Return a task factory or "None" if the default one is in use.


打开网络连接
------------

coroutine loop.create_connection(protocol_factory, host=None, port=None, *, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None, ssl_handshake_timeout=None, happy_eyeballs_delay=None, interleave=None)

   Open a streaming transport connection to a given address specified
   by *host* and *port*.

   The socket family can be either "AF_INET" or "AF_INET6" depending
   on *host* (or the *family* argument, if provided).

   The socket type will be "SOCK_STREAM".

   *protocol_factory* must be a callable returning an asyncio protocol
   implementation.

   这个方法会尝试在后台创建连接。当创建成功，返回 "(transport,
   protocol)" 组合。

   底层操作的大致的执行顺序是这样的：

   1. The connection is established and a transport is created for it.

   2. *protocol_factory* is called without arguments and is expected
      to return a protocol instance.

   3. The protocol instance is coupled with the transport by calling
      its "connection_made()" method.

   4. 成功时返回一个 "(transport, protocol)" 元组。

   被创建的传输对象是一个实现相关的双向流。

   其他参数：

   * *ssl*: if given and not false, a SSL/TLS transport is created (by
     default a plain TCP transport is created).  If *ssl* is a
     "ssl.SSLContext" object, this context is used to create the
     transport; if *ssl* is "True", a default context returned from
     "ssl.create_default_context()" is used.

     也參考: SSL/TLS security considerations

   * *server_hostname* sets or overrides the hostname that the target
     server's certificate will be matched against.  Should only be
     passed if *ssl* is not "None".  By default the value of the
     *host* argument is used.  If *host* is empty, there is no default
     and you must pass a value for *server_hostname*.  If
     *server_hostname* is an empty string, hostname matching is
     disabled (which is a serious security risk, allowing for
     potential man-in-the-middle attacks).

   * *family*, *proto*, *flags*  是可选的地址族、协议和标志，它们会被
     传递给 getaddrinfo() 来对 *host* 进行解析。如果要指定的话，这些都
     应该是来自于 "socket" 模块的对应常量。

   * *happy_eyeballs_delay*, if given, enables Happy Eyeballs for this
     connection. It should be a floating-point number representing the
     amount of time in seconds to wait for a connection attempt to
     complete, before starting the next attempt in parallel. This is
     the "Connection Attempt Delay" as defined in **RFC 8305**. A
     sensible default value recommended by the RFC is "0.25" (250
     milliseconds).

   * *interleave* controls address reordering when a host name
     resolves to multiple IP addresses. If "0" or unspecified, no
     reordering is done, and addresses are tried in the order returned
     by "getaddrinfo()". If a positive integer is specified, the
     addresses are interleaved by address family, and the given
     integer is interpreted as "First Address Family Count" as defined
     in **RFC 8305**. The default is "0" if *happy_eyeballs_delay* is
     not specified, and "1" if it is.

   * *sock*, if given, should be an existing, already connected
     "socket.socket" object to be used by the transport. If *sock* is
     given, none of *host*, *port*, *family*, *proto*, *flags*,
     *happy_eyeballs_delay*, *interleave* and *local_addr* should be
     specified.

   * *local_addr*, if given, is a "(local_host, local_port)" tuple
     used to bind the socket to locally.  The *local_host* and
     *local_port* are looked up using "getaddrinfo()", similarly to
     *host* and *port*.

   * *ssl_handshake_timeout* is (for a TLS connection) the time in
     seconds to wait for the TLS handshake to complete before aborting
     the connection. "60.0" seconds if "None" (default).

   3.8 版新加入: Added the *happy_eyeballs_delay* and *interleave*
   parameters.Happy Eyeballs Algorithm: Success with Dual-Stack Hosts.
   When a server's IPv4 path and protocol are working, but the
   server's IPv6 path and protocol are not working, a dual-stack
   client application experiences significant connection delay
   compared to an IPv4-only client.  This is undesirable because it
   causes the dual- stack client to have a worse user experience.
   This document specifies requirements for algorithms that reduce
   this user-visible delay and provides an algorithm.For more
   information: https://tools.ietf.org/html/rfc6555

   3.7 版新加入: *ssl_handshake_timeout* 形参。

   3.6 版更變: The socket option "TCP_NODELAY" is set by default for
   all TCP connections.

   3.5 版更變: "ProactorEventLoop" 类中添加 SSL/TLS 支持。

   也參考:

     The "open_connection()" function is a high-level alternative API.
     It returns a pair of ("StreamReader", "StreamWriter") that can be
     used directly in async/await code.

coroutine loop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0, reuse_address=None, reuse_port=None, allow_broadcast=None, sock=None)

   備註:

     形参 *reuse_address* 已不再受支持，因为使用 "SO_REUSEADDR" 会对
     UDP 造成显著的安全问题。 显式地传入 "reuse_address=True" 将会引发
     异常。When multiple processes with differing UIDs assign sockets
     to an identical UDP socket address with "SO_REUSEADDR", incoming
     packets can become randomly distributed among the sockets.对于受
     支持的平台，*reuse_port* 可以被用作类似功能的替代。 通过
     *reuse_port* 将改用 "SO_REUSEPORT"，它能够防止具有不同 UID 的进程
     将套接字赋给相同的套接字地址。

   创建一个数据报连接。

   The socket family can be either "AF_INET", "AF_INET6", or
   "AF_UNIX", depending on *host* (or the *family* argument, if
   provided).

   socket类型将是 "SOCK_DGRAM" 。

   *protocol_factory* must be a callable returning a protocol
   implementation.

   A tuple of "(transport, protocol)" is returned on success.

   其他参数：

   * *local_addr*，如果指定的话，就是一个 "(local_host, local_port)"
     元组，用于在本地绑定套接字。 *local_host* 和 *local_port* 是使用
     "getaddrinfo()" 来查找的。

   * *remote_addr*，如果指定的话，就是一个 "(remote_host,
     remote_port)" 元组，用于同一个远程地址连接。*remote_host* 和
     *remote_port* 是使用 "getaddrinfo()" 来查找的。

   * *family*, *proto*, *flags* 是可选的地址族，协议和标志，其会被传递
     给 "getaddrinfo()" 来完成 *host* 的解析。如果要指定的话，这些都应
     该是来自于 "socket" 模块的对应常量。

   * *reuse_port* tells the kernel to allow this endpoint to be bound
     to the same port as other existing endpoints are bound to, so
     long as they all set this flag when being created. This option is
     not supported on Windows and some Unixes. If the "SO_REUSEPORT"
     constant is not defined then this capability is unsupported.

   * *allow_broadcast* 告知内核允许此端点向广播地址发送消息。

   * *sock* 可选择通过指定此值用于使用一个预先存在的，已经处于连接状态
     的 "socket.socket" 对象，并将其提供给此传输对象使用。如果指定了这
     个值， *local_addr* 和 *remote_addr* 就应该被忽略 (必须为 "None")
     。

   参见 UDP echo 客户端协议  和 UDP echo 服务端协议 的例子。

   3.4.4 版更變: 添加了 *family*, *proto*, *flags*, *reuse_address*,
   *reuse_port*, *allow_broadcast* 和 *sock* 等参数。

   3.8.1 版更變: The *reuse_address* parameter is no longer supported
   due to security concerns.

   3.8 版更變: 添加WIndows的支持。

coroutine loop.create_unix_connection(protocol_factory, path=None, *, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None)

   创建Unix 连接

   The socket family will be "AF_UNIX"; socket type will be
   "SOCK_STREAM".

   A tuple of "(transport, protocol)" is returned on success.

   *path* is the name of a Unix domain socket and is required, unless
   a *sock* parameter is specified.  Abstract Unix sockets, "str",
   "bytes", and "Path" paths are supported.

   See the documentation of the "loop.create_connection()" method for
   information about arguments to this method.

   可用性: Unix。

   3.7 版新加入: *ssl_handshake_timeout* 形参。

   3.7 版更變: *path* 形参现在可以是 *path-like object* 对象。


创建网络服务
------------

coroutine loop.create_server(protocol_factory, host=None, port=None, *, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None, reuse_port=None, ssl_handshake_timeout=None, start_serving=True)

   创建TCP服务 (socket 类型 "SOCK_STREAM" ) 监听 *host* 地址的 *port*
   端口。

   返回一个 "Server" 对象。

   参数：

   * *protocol_factory* must be a callable returning a protocol
     implementation.

   * The *host* parameter can be set to several types which determine
     where the server would be listening:

     * If *host* is a string, the TCP server is bound to a single
       network interface specified by *host*.

     * If *host* is a sequence of strings, the TCP server is bound to
       all network interfaces specified by the sequence.

     * If *host* is an empty string or "None", all interfaces are
       assumed and a list of multiple sockets will be returned (most
       likely one for IPv4 and another one for IPv6).

   * *family* can be set to either "socket.AF_INET" or "AF_INET6" to
     force the socket to use IPv4 or IPv6. If not set, the *family*
     will be determined from host name (defaults to "AF_UNSPEC").

   * *flags* 是用于 "getaddrinfo()" 的位掩码。

   * *sock* can optionally be specified in order to use a preexisting
     socket object. If specified, *host* and *port* must not be
     specified.

   * *backlog* 是传递给 "listen()" 的最大排队连接的数量（默认为100）。

   * *ssl* can be set to an "SSLContext" instance to enable TLS over
     the accepted connections.

   * *reuse_address* tells the kernel to reuse a local socket in
     "TIME_WAIT" state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to "True" on
     Unix.

   * *reuse_port* 告知内核，只要在创建的时候都设置了这个标志，就允许此
     端点绑定到其它端点列表所绑定的同样的端口上。这个选项在 Windows 上
     是不支持的。

   * *ssl_handshake_timeout* is (for a TLS server) the time in seconds
     to wait for the TLS handshake to complete before aborting the
     connection. "60.0" seconds if "None" (default).

   * *start_serving* 设置成 "True" (默认值) 会导致创建server并立即开始
     接受连接。设置成 "False" ，用户需要等待 "Server.start_serving()"
     或者 "Server.serve_forever()" 以使server开始接受连接。

   3.7 版新加入: Added *ssl_handshake_timeout* and *start_serving*
   parameters.

   3.6 版更變: The socket option "TCP_NODELAY" is set by default for
   all TCP connections.

   3.5 版更變: "ProactorEventLoop" 类中添加 SSL/TLS 支持。

   3.5.1 版更變: The *host* parameter can be a sequence of strings.

   也參考:

     The "start_server()" function is a higher-level alternative API
     that returns a pair of "StreamReader" and "StreamWriter" that can
     be used in an async/await code.

coroutine loop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, start_serving=True)

   Similar to "loop.create_server()" but works with the "AF_UNIX"
   socket family.

   *path* is the name of a Unix domain socket, and is required, unless
   a *sock* argument is provided.  Abstract Unix sockets, "str",
   "bytes", and "Path" paths are supported.

   See the documentation of the "loop.create_server()" method for
   information about arguments to this method.

   可用性: Unix。

   3.7 版新加入: The *ssl_handshake_timeout* and *start_serving*
   parameters.

   3.7 版更變: *path* 形参现在可以是 "Path" 对象。

coroutine loop.connect_accepted_socket(protocol_factory, sock, *, ssl=None, ssl_handshake_timeout=None)

   Wrap an already accepted connection into a transport/protocol pair.

   This method can be used by servers that accept connections outside
   of asyncio but that use asyncio to handle them.

   参数：

   * *protocol_factory* must be a callable returning a protocol
     implementation.

   * *sock* is a preexisting socket object returned from
     "socket.accept".

   * *ssl* 可被设置为一个 "SSLContext" 以在接受的连接上启用 SSL。

   * *ssl_handshake_timeout* 是(为一个SSL连接)在中止连接前，等待SSL握
     手完成的时间【单位秒】。如果为 "None" (缺省) 则是 "60.0" 秒。

   返回一个 "(transport, protocol)" 对。

   3.7 版新加入: *ssl_handshake_timeout* 形参。

   3.5.3 版新加入.


传输文件
--------

coroutine loop.sendfile(transport, file, offset=0, count=None, *, fallback=True)

   Send a *file* over a *transport*.  Return the total number of bytes
   sent.

   如果可用的化，该方法将使用高性能的 "os.sendfile()" 。

   *file* 必须是个二进制模式打开的常规文件对象。

   *offset* tells from where to start reading the file. If specified,
   *count* is the total number of bytes to transmit as opposed to
   sending the file until EOF is reached. File position is always
   updated, even when this method raises an error, and "file.tell()"
   can be used to obtain the actual number of bytes sent.

   *fallback* set to "True" makes asyncio to manually read and send
   the file when the platform does not support the sendfile system
   call (e.g. Windows or SSL socket on Unix).

   Raise "SendfileNotAvailableError" if the system does not support
   the *sendfile* syscall and *fallback* is "False".

   3.7 版新加入.


TLS 升级
--------

coroutine loop.start_tls(transport, protocol, sslcontext, *, server_side=False, server_hostname=None, ssl_handshake_timeout=None)

   Upgrade an existing transport-based connection to TLS.

   Return a new transport instance, that the *protocol* must start
   using immediately after the *await*.  The *transport* instance
   passed to the *start_tls* method should never be used again.

   参数：

   * *transport* and *protocol* instances that methods like
     "create_server()" and "create_connection()" return.

   * *sslcontext* ：一个已经配置好的 "SSLContext" 实例。

   * *server_side* pass "True" when a server-side connection is being
     upgraded (like the one created by "create_server()").

   * *server_hostname* ：设置或者覆盖目标服务器证书中相对应的主机名。

   * *ssl_handshake_timeout* is (for a TLS connection) the time in
     seconds to wait for the TLS handshake to complete before aborting
     the connection. "60.0" seconds if "None" (default).

   3.7 版新加入.


监控文件描述符
--------------

loop.add_reader(fd, callback, *args)

   开始监视 *fd* 文件描述符以获取读取的可用性，一旦 *fd* 可用于读取，
   使用指定的参数调用 *callback* 。

loop.remove_reader(fd)

   停止对文件描述符 *fd* 读取可用性的监视。

loop.add_writer(fd, callback, *args)

   开始监视 *fd* 文件描述符的写入可用性，一旦 *fd* 可用于写入，使用指
   定的参数调用 *callback* 。

   使用 "functools.partial()" 传递关键字参数  给 *callback*.

loop.remove_writer(fd)

   停止对文件描述符 *fd* 的写入可用性监视。

See also Platform Support section for some limitations of these
methods.


直接使用 socket 对象
--------------------

In general, protocol implementations that use transport-based APIs
such as "loop.create_connection()" and "loop.create_server()" are
faster than implementations that work with sockets directly. However,
there are some use cases when performance is not critical, and working
with "socket" objects directly is more convenient.

coroutine loop.sock_recv(sock, nbytes)

   Receive up to *nbytes* from *sock*.  Asynchronous version of
   "socket.recv()".

   返回接收到的数据【bytes对象类型】。

   *sock* 必须是个非阻塞socket。

   3.7 版更變: Even though this method was always documented as a
   coroutine method, releases before Python 3.7 returned a "Future".
   Since Python 3.7 this is an "async def" method.

coroutine loop.sock_recv_into(sock, buf)

   Receive data from *sock* into the *buf* buffer.  Modeled after the
   blocking "socket.recv_into()" method.

   返回写入缓冲区的字节数。

   *sock* 必须是个非阻塞socket。

   3.7 版新加入.

coroutine loop.sock_sendall(sock, data)

   Send *data* to the *sock* socket. Asynchronous version of
   "socket.sendall()".

   This method continues to send to the socket until either all data
   in *data* has been sent or an error occurs.  "None" is returned on
   success.  On error, an exception is raised. Additionally, there is
   no way to determine how much data, if any, was successfully
   processed by the receiving end of the connection.

   *sock* 必须是个非阻塞socket。

   3.7 版更變: 虽然这个方法一直被标记为协程方法。但是，Python 3.7 之前
   ，该方法返回 "Future" ，从Python 3.7 开始，这个方法是 "async def"
   方法。

coroutine loop.sock_connect(sock, address)

   Connect *sock* to a remote socket at *address*.

   Asynchronous version of "socket.connect()".

   *sock* 必须是个非阻塞socket。

   3.5.2 版更變: "address" no longer needs to be resolved.
   "sock_connect" will try to check if the *address* is already
   resolved by calling "socket.inet_pton()".  If not,
   "loop.getaddrinfo()" will be used to resolve the *address*.

   也參考: "loop.create_connection()" 和  "asyncio.open_connection()" 。

coroutine loop.sock_accept(sock)

   Accept a connection.  Modeled after the blocking "socket.accept()"
   method.

   此 *scoket* 必须绑定到一个地址上并且监听连接。返回值是一个 "(conn,
   address)" 对，其中 *conn* 是一个 *新*的套接字对象，用于在此连接上收
   发数据，*address* 是连接的另一端的套接字所绑定的地址。

   *sock* 必须是个非阻塞socket。

   3.7 版更變: 虽然这个方法一直被标记为协程方法。但是，Python 3.7 之前
   ，该方法返回 "Future" ，从Python 3.7 开始，这个方法是 "async def"
   方法。

   也參考: "loop.create_server()" and "start_server()".

coroutine loop.sock_sendfile(sock, file, offset=0, count=None, *, fallback=True)

   Send a file using high-performance "os.sendfile" if possible.
   Return the total number of bytes sent.

   Asynchronous version of "socket.sendfile()".

   *sock* must be a non-blocking "socket.SOCK_STREAM" "socket".

   *file* 必须是个用二进制方式打开的常规文件对象。

   *offset* tells from where to start reading the file. If specified,
   *count* is the total number of bytes to transmit as opposed to
   sending the file until EOF is reached. File position is always
   updated, even when this method raises an error, and "file.tell()"
   can be used to obtain the actual number of bytes sent.

   *fallback*, when set to "True", makes asyncio manually read and
   send the file when the platform does not support the sendfile
   syscall (e.g. Windows or SSL socket on Unix).

   如果系统不支持 *sendfile* 并且 *fallback* 为 "False" ，引发
   "SendfileNotAvailableError" 异常。

   *sock* 必须是个非阻塞socket。

   3.7 版新加入.


DNS
---

coroutine loop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

   异步版的 "socket.getaddrinfo()" 。

coroutine loop.getnameinfo(sockaddr, flags=0)

   异步版的 "socket.getnameinfo()" 。

3.7 版更變: *getaddrinfo* 和 *getnameinfo* 方法一直被标记返回一个协程
，但是Python 3.7之前，实际返回的是 "asyncio.Future" 对象。从Python 3.7
开始，这两个方法是协程。


使用管道
--------

coroutine loop.connect_read_pipe(protocol_factory, pipe)

   Register the read end of *pipe* in the event loop.

   *protocol_factory* must be a callable returning an asyncio protocol
   implementation.

   *pipe* 是个 *类似文件型对象*.

   Return pair "(transport, protocol)", where *transport* supports the
   "ReadTransport" interface and *protocol* is an object instantiated
   by the *protocol_factory*.

   使用 "SelectorEventLoop" 事件循环， *pipe* 被设置为非阻塞模式。

coroutine loop.connect_write_pipe(protocol_factory, pipe)

   Register the write end of *pipe* in the event loop.

   *protocol_factory* must be a callable returning an asyncio protocol
   implementation.

   *pipe* 是个 *类似文件型对象*.

   Return pair "(transport, protocol)", where *transport* supports
   "WriteTransport" interface and *protocol* is an object instantiated
   by the *protocol_factory*.

   使用 "SelectorEventLoop" 事件循环， *pipe* 被设置为非阻塞模式。

備註:

  "SelectorEventLoop" does not support the above methods on Windows.
  Use "ProactorEventLoop" instead for Windows.

也參考:

  The "loop.subprocess_exec()" and "loop.subprocess_shell()" methods.


Unix 信号
---------

loop.add_signal_handler(signum, callback, *args)

   设置 *callback* 作为 *signum* 信号的处理程序。

   The callback will be invoked by *loop*, along with other queued
   callbacks and runnable coroutines of that event loop. Unlike signal
   handlers registered using "signal.signal()", a callback registered
   with this function is allowed to interact with the event loop.

   如果信号数字非法或者不可捕获，就抛出一个 "ValueError" 。如果建立处
   理器的过程中出现问题，会抛出一个 "RuntimeError" 。

   使用 "functools.partial()" 传递关键字参数  给 *callback*.

   和 "signal.signal()" 一样，这个函数只能在主线程中调用。

loop.remove_signal_handler(sig)

   移除 *sig* 信号的处理程序。

   Return "True" if the signal handler was removed, or "False" if no
   handler was set for the given signal.

   可用性: Unix。

也參考: "signal" 模块。


在线程或者进程池中执行代码。
----------------------------

awaitable loop.run_in_executor(executor, func, *args)

   安排在指定的执行器中调用 *func* 。

   The *executor* argument should be an "concurrent.futures.Executor"
   instance. The default executor is used if *executor* is "None".

   示例:

      import asyncio
      import concurrent.futures

      def blocking_io():
          # File operations (such as logging) can block the
          # event loop: run them in a thread pool.
          with open('/dev/urandom', 'rb') as f:
              return f.read(100)

      def cpu_bound():
          # CPU-bound operations will block the event loop:
          # in general it is preferable to run them in a
          # process pool.
          return sum(i * i for i in range(10 ** 7))

      async def main():
          loop = asyncio.get_running_loop()

          ## Options:

          # 1. Run in the default loop's executor:
          result = await loop.run_in_executor(
              None, blocking_io)
          print('default thread pool', result)

          # 2. Run in a custom thread pool:
          with concurrent.futures.ThreadPoolExecutor() as pool:
              result = await loop.run_in_executor(
                  pool, blocking_io)
              print('custom thread pool', result)

          # 3. Run in a custom process pool:
          with concurrent.futures.ProcessPoolExecutor() as pool:
              result = await loop.run_in_executor(
                  pool, cpu_bound)
              print('custom process pool', result)

      asyncio.run(main())

   这个方法返回一个 "asyncio.Future" 对象。

   使用 "functools.partial()" 传递关键字参数  给 *func* 。

   3.5.3 版更變: "loop.run_in_executor()" no longer configures the
   "max_workers" of the thread pool executor it creates, instead
   leaving it up to the thread pool executor ("ThreadPoolExecutor") to
   set the default.

loop.set_default_executor(executor)

   Set *executor* as the default executor used by "run_in_executor()".
   *executor* should be an instance of "ThreadPoolExecutor".

   3.8 版後已棄用: Using an executor that is not an instance of
   "ThreadPoolExecutor" is deprecated and will trigger an error in
   Python 3.9.

   *executor* 必须是个 "concurrent.futures.ThreadPoolExecutor" 的实例
   。


错误处理API
-----------

允许自定义事件循环中如何去处理异常。

loop.set_exception_handler(handler)

   将 *handler* 设置为新的事件循环异常处理器。

   If *handler* is "None", the default exception handler will be set.
   Otherwise, *handler* must be a callable with the signature matching
   "(loop, context)", where "loop" is a reference to the active event
   loop, and "context" is a "dict" object containing the details of
   the exception (see "call_exception_handler()" documentation for
   details about context).

loop.get_exception_handler()

   返回当前的异常处理器，如果没有设置异常处理器，则返回 "None" 。

   3.5.2 版新加入.

loop.default_exception_handler(context)

   默认的异常处理器。

   This is called when an exception occurs and no exception handler is
   set. This can be called by a custom exception handler that wants to
   defer to the default handler behavior.

   *context* 参数和 "call_exception_handler()" 中的同名参数完全相同。

loop.call_exception_handler(context)

   调用当前事件循环的异常处理器。

   *context* 是个包含下列键的 "dict" 对象(未来版本的Python可能会引入新
   键)：

   * 'message': 错误消息；

   * 'exception' （可选）: 异常对象；

   * 'future' （可选）:  "asyncio.Future" 实例；

   * 'handle' （可选）: "asyncio.Handle" 实例；

   * 'protocol' （可选）: Protocol 实例；

   * 'transport' （可选）: Transport 实例；

   * 'socket' （可选）： "socket.socket" 实例。

   備註:

     This method should not be overloaded in subclassed event loops.
     For custom exception handling, use the "set_exception_handler()"
     method.


开启调试模式
------------

loop.get_debug()

   获取事件循环调试模式设置（"bool"）。

   如果环境变量 "PYTHONASYNCIODEBUG" 是一个非空字符串，就返回 "True"
   ，否则就返回 "False" 。

loop.set_debug(enabled: bool)

   设置事件循环的调试模式。

   3.7 版更變: The new "-X dev" command line option can now also be
   used to enable the debug mode.

也參考: debug mode of asyncio.


运行子进程
----------

Methods described in this subsections are low-level.  In regular
async/await code consider using the high-level
"asyncio.create_subprocess_shell()" and
"asyncio.create_subprocess_exec()" convenience functions instead.

備註:

  The default asyncio event loop on **Windows** does not support
  subprocesses. See Subprocess Support on Windows for details.

coroutine loop.subprocess_exec(protocol_factory, *args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kwargs)

   用 *args* 指定的一个或者多个字符串型参数创建一个子进程。

   *args* 必须是个由下列形式的字符串组成的列表：

   * "str";

   * 或者由 文件熊编码 编码的 "bytes" 。

   The first string specifies the program executable, and the
   remaining strings specify the arguments.  Together, string
   arguments form the "argv" of the program.

   This is similar to the standard library "subprocess.Popen" class
   called with "shell=False" and the list of strings passed as the
   first argument; however, where "Popen" takes a single argument
   which is list of strings, *subprocess_exec* takes multiple string
   arguments.

   The *protocol_factory* must be a callable returning a subclass of
   the "asyncio.SubprocessProtocol" class.

   其他参数：

   * *stdin* can be any of these:

     * a file-like object representing a pipe to be connected to the
       subprocess's standard input stream using "connect_write_pipe()"

     * the "subprocess.PIPE" constant (default) which will create a
       new pipe and connect it,

     * the value "None" which will make the subprocess inherit the
       file descriptor from this process

     * the "subprocess.DEVNULL" constant which indicates that the
       special "os.devnull" file will be used

   * *stdout* can be any of these:

     * a file-like object representing a pipe to be connected to the
       subprocess's standard output stream using
       "connect_write_pipe()"

     * the "subprocess.PIPE" constant (default) which will create a
       new pipe and connect it,

     * the value "None" which will make the subprocess inherit the
       file descriptor from this process

     * the "subprocess.DEVNULL" constant which indicates that the
       special "os.devnull" file will be used

   * *stderr* can be any of these:

     * a file-like object representing a pipe to be connected to the
       subprocess's standard error stream using "connect_write_pipe()"

     * the "subprocess.PIPE" constant (default) which will create a
       new pipe and connect it,

     * the value "None" which will make the subprocess inherit the
       file descriptor from this process

     * the "subprocess.DEVNULL" constant which indicates that the
       special "os.devnull" file will be used

     * the "subprocess.STDOUT" constant which will connect the
       standard error stream to the process' standard output stream

   * All other keyword arguments are passed to "subprocess.Popen"
     without interpretation, except for *bufsize*,
     *universal_newlines*, *shell*, *text*, *encoding* and *errors*,
     which should not be specified at all.

     The "asyncio" subprocess API does not support decoding the
     streams as text. "bytes.decode()" can be used to convert the
     bytes returned from the stream to text.

   其他参数的文档，请参阅 "subprocess.Popen" 类的构造函数。

   Returns a pair of "(transport, protocol)", where *transport*
   conforms to the "asyncio.SubprocessTransport" base class and
   *protocol* is an object instantiated by the *protocol_factory*.

coroutine loop.subprocess_shell(protocol_factory, cmd, *, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kwargs)

   Create a subprocess from *cmd*, which can be a "str" or a "bytes"
   string encoded to the filesystem encoding, using the platform's
   "shell" syntax.

   这类似与用 "shell=True" 调用标准库的 "subprocess.Popen" 类。

   The *protocol_factory* must be a callable returning a subclass of
   the "SubprocessProtocol" class.

   See "subprocess_exec()" for more details about the remaining
   arguments.

   Returns a pair of "(transport, protocol)", where *transport*
   conforms to the "SubprocessTransport" base class and *protocol* is
   an object instantiated by the *protocol_factory*.

備註:

  It is the application's responsibility to ensure that all whitespace
  and special characters are quoted appropriately to avoid shell
  injection vulnerabilities. The "shlex.quote()" function can be used
  to properly escape whitespace and special characters in strings that
  are going to be used to construct shell commands.


回调处理
========

class asyncio.Handle

   A callback wrapper object returned by "loop.call_soon()",
   "loop.call_soon_threadsafe()".

   cancel()

      Cancel the callback.  If the callback has already been canceled
      or executed, this method has no effect.

   cancelled()

      Return "True" if the callback was cancelled.

      3.7 版新加入.

class asyncio.TimerHandle

   A callback wrapper object returned by "loop.call_later()", and
   "loop.call_at()".

   This class is a subclass of "Handle".

   when()

      Return a scheduled callback time as "float" seconds.

      The time is an absolute timestamp, using the same time reference
      as "loop.time()".

      3.7 版新加入.


Server Objects
==============

Server objects are created by "loop.create_server()",
"loop.create_unix_server()", "start_server()", and
"start_unix_server()" functions.

Do not instantiate the class directly.

class asyncio.Server

   *Server* 对象是异步上下文管理器。当用于 "async with" 语句时，异步上
   下文管理器可以确保 Server 对象被关闭，并且在 "async with" 语句完成
   后，不接受新的连接。

      srv = await loop.create_server(...)

      async with srv:
          # some code

      # At this point, srv is closed and no longer accepts new connections.

   3.7 版更變: Python3.7 开始，Server 对象是一个异步上下文管理器。

   close()

      停止服务：关闭监听的套接字并且设置 "sockets" 属性为 "None" 。

      用于表示已经连进来的客户端连接会保持打开的状态。

      服务器是被异步关闭的，使用 "wait_closed()" 协程来等待服务器关闭
      。

   get_loop()

      Return the event loop associated with the server object.

      3.7 版新加入.

   coroutine start_serving()

      开始接受连接。

      这个方法是幂等的【相同参数重复执行，能获得相同的结果】，所以此方
      法能在服务已经运行的时候调用。

      The *start_serving* keyword-only parameter to
      "loop.create_server()" and "asyncio.start_server()" allows
      creating a Server object that is not accepting connections
      initially.  In this case "Server.start_serving()", or
      "Server.serve_forever()" can be used to make the Server start
      accepting connections.

      3.7 版新加入.

   coroutine serve_forever()

      开始接受连接，直到协程被取消。 "serve_forever" 任务的取消将导致
      服务器被关闭。

      如果服务器已经在接受连接了，这个方法可以被调用。每个 *Server* 对
      象，仅能有一个 "serve_forever" 任务。

      示例:

         async def client_connected(reader, writer):
             # Communicate with the client with
             # reader/writer streams.  For example:
             await reader.readline()

         async def main(host, port):
             srv = await asyncio.start_server(
                 client_connected, host, port)
             await srv.serve_forever()

         asyncio.run(main('127.0.0.1', 0))

      3.7 版新加入.

   is_serving()

      如果服务器正在接受新连接的状态，返回 "True" 。

      3.7 版新加入.

   coroutine wait_closed()

      等待 "close()" 方法执行完毕。

   sockets

      List of "socket.socket" objects the server is listening on.

      3.7 版更變: Prior to Python 3.7 "Server.sockets" used to return
      an internal list of server sockets directly.  In 3.7 a copy of
      that list is returned.


事件循环实现
============

asyncio ships with two different event loop implementations:
"SelectorEventLoop" and "ProactorEventLoop".

By default asyncio is configured to use "SelectorEventLoop" on Unix
and "ProactorEventLoop" on Windows.

class asyncio.SelectorEventLoop

   An event loop based on the "selectors" module.

   Uses the most efficient *selector* available for the given
   platform.  It is also possible to manually configure the exact
   selector implementation to be used:

      import asyncio
      import selectors

      selector = selectors.SelectSelector()
      loop = asyncio.SelectorEventLoop(selector)
      asyncio.set_event_loop(loop)

   可用性: Unix, Windows。

class asyncio.ProactorEventLoop

   用 "I/O Completion Ports" (IOCP) 构建的专为Windows 的事件循环。

   可用性: Windows。

   也參考: MSDN documentation on I/O Completion Ports.

class asyncio.AbstractEventLoop

   Abstract base class for asyncio-compliant event loops.

   The Event Loop Methods section lists all methods that an
   alternative implementation of "AbstractEventLoop" should have
   defined.


例子
====

Note that all examples in this section **purposefully** show how to
use the low-level event loop APIs, such as "loop.run_forever()" and
"loop.call_soon()".  Modern asyncio applications rarely need to be
written this way; consider using the high-level functions like
"asyncio.run()".


call_soon() 的 Hello World 示例。
---------------------------------

An example using the "loop.call_soon()" method to schedule a callback.
The callback displays ""Hello World"" and then stops the event loop:

   import asyncio

   def hello_world(loop):
       """A callback to print 'Hello World' and stop the event loop"""
       print('Hello World')
       loop.stop()

   loop = asyncio.get_event_loop()

   # Schedule a call to hello_world()
   loop.call_soon(hello_world, loop)

   # Blocking call interrupted by loop.stop()
   try:
       loop.run_forever()
   finally:
       loop.close()

也參考:

  A similar Hello World example created with a coroutine and the
  "run()" function.


使用 call_later() 来展示当前的日期
----------------------------------

An example of a callback displaying the current date every second. The
callback uses the "loop.call_later()" method to reschedule itself
after 5 seconds, and then stops the event loop:

   import asyncio
   import datetime

   def display_date(end_time, loop):
       print(datetime.datetime.now())
       if (loop.time() + 1.0) < end_time:
           loop.call_later(1, display_date, end_time, loop)
       else:
           loop.stop()

   loop = asyncio.get_event_loop()

   # Schedule the first call to display_date()
   end_time = loop.time() + 5.0
   loop.call_soon(display_date, end_time, loop)

   # Blocking call interrupted by loop.stop()
   try:
       loop.run_forever()
   finally:
       loop.close()

也參考:

  A similar current date example created with a coroutine and the
  "run()" function.


监控一个文件描述符的读事件
--------------------------

使用 "loop.add_reader()" 方法，等到文件描述符收到一些数据，然后关闭事
件循环：

   import asyncio
   from socket import socketpair

   # Create a pair of connected file descriptors
   rsock, wsock = socketpair()

   loop = asyncio.get_event_loop()

   def reader():
       data = rsock.recv(100)
       print("Received:", data.decode())

       # We are done: unregister the file descriptor
       loop.remove_reader(rsock)

       # Stop the event loop
       loop.stop()

   # Register the file descriptor for read event
   loop.add_reader(rsock, reader)

   # Simulate the reception of data from the network
   loop.call_soon(wsock.send, 'abc'.encode())

   try:
       # Run the event loop
       loop.run_forever()
   finally:
       # We are done. Close sockets and the event loop.
       rsock.close()
       wsock.close()
       loop.close()

也參考:

  * A similar example using transports, protocols, and the
    "loop.create_connection()" method.

  * Another similar example using the high-level
    "asyncio.open_connection()" function and streams.


为SIGINT和SIGTERM设置信号处理器
-------------------------------

(This "signals" example only works on Unix.)

Register handlers for signals "SIGINT" and "SIGTERM" using the
"loop.add_signal_handler()" method:

   import asyncio
   import functools
   import os
   import signal

   def ask_exit(signame, loop):
       print("got signal %s: exit" % signame)
       loop.stop()

   async def main():
       loop = asyncio.get_running_loop()

       for signame in {'SIGINT', 'SIGTERM'}:
           loop.add_signal_handler(
               getattr(signal, signame),
               functools.partial(ask_exit, signame, loop))

       await asyncio.sleep(3600)

   print("Event loop running for 1 hour, press Ctrl+C to interrupt.")
   print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.")

   asyncio.run(main())
