# 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。

内容

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)

  安排在下一次事件循环的迭代中使用 args 参数调用 callback 。

  回调按其注册顺序被调用。每个回调仅被调用一次。

  可选的仅关键字型参数 context 允许为要运行的 callback 指定一个自定义 `contextvars.Context` 。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。

  查看 并发和多线程 章节的文档。

3.7 版更變: 仅用于关键字形参的参数 context 已经被添加。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()` 。

  可选的仅关键字型参数 context 允许为要运行的 callback 指定一个自定义 `contextvars.Context` 。如果没有提供 context ，则使用当前上下文。

  3.7 版更變: 仅用于关键字形参的参数 context 已经被添加。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。

  这个函数的行为与 `call_later()` 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: 仅用于关键字形参的参数 context 已经被添加。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。

  三方的事件循环可以使用它们自己定义的 `Task` 类的子类来实现互操作性。这个例子里，返回值的类型是 `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: Added the `name` parameter.

`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。

  基本操作的时间顺序如下：

  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。

  创建的transport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安全事项

  - 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，通过传递给 getaddrinfo() 来解析 host。如果给出，这些应该都是来自 `socket` 模块相应的常量的整数。

  - 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.

  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

  The parameter reuse_address is no longer supported, as using `SO_REUSEADDR` poses a significant security concern for UDP. Explicitly passing `reuse_address=True` will raise an exception.

  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.

  For supported platforms, reuse_port can be used as a replacement for similar functionality. With reuse_port, `SO_REUSEPORT` is used instead, which specifically prevents processes with differing UIDs from assigning sockets to the same socket address.

  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状态（`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.

## 示例

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())