# 事件循环

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 新版功能.

`asyncio.``get_event_loop`()

  获取当前事件循环。 如果当前 OS 线程没有设置当前事件循环并且 `set_event_loop()` 还没有被调用，asyncio 将创建一个新的事件循环并将其设置为当前循环。

  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:

- The Event Loop Methods section is the reference documentation of the event loop APIs;
- The Callback Handles section documents the `Handle` and `TimerHandle` instances which are returned from scheduling methods such as `loop.call_soon()` and `loop.call_later()`;
- The Server Objects section documents types returned from event loop methods like `loop.create_server()`;
- The Event Loop Implementations section documents the `SelectorEventLoop` and `ProactorEventLoop` classes;
- The Examples section showcases how to work with some event loop APIs.

## Event Loop Methods

Event loops have low-level APIs for the following:

- Running and stopping the loop
- Scheduling callbacks
- Scheduling delayed callbacks
- Creating Futures and Tasks
- Opening network connections
- Creating network servers
- Transferring files
- TLS Upgrade
- Watching file descriptors
- Working with socket objects directly
- DNS
- Working with pipes
- Unix signals
- Executing code in thread or process pools
- 错误处理API
- Enabling debug mode
- Running Subprocesses

### Running and stopping the loop

`loop.``run_until_complete`(future)

  Run until the future (an instance of `Future`) has completed.

  If the argument is a coroutine object it is implicitly scheduled to run as a `asyncio.Task`.

  Return the Future's result or raise its exception.

`loop.``run_forever`()

  Run the event loop until `stop()` is called.

  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`()

  Stop the event loop.

`loop.``is_running`()

  Return `True` if the event loop is currently running.

`loop.``is_closed`()

  Return `True` if the event loop was closed.

`loop.``close`()

  Close the event loop.

  The loop must not be running when this function is called. Any pending callbacks will be discarded.

  This method clears all queues and shuts down the executor, but does not wait for the executor to finish.

  This method is idempotent and irreversible. No other methods should be called after the event loop is closed.

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.

  Note that there is no need to call this function when `asyncio.run()` is used.

  示例:

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

  3.6 新版功能.

### Scheduling callbacks

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

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

  Callbacks are called in the order in which they are registered. Each callback will be called exactly once.

  An optional keyword-only context argument allows specifying a custom `contextvars.Context` for the callback to run in. The current context is used when no context is provided.

  An instance of `asyncio.Handle` is returned, which can be used later to cancel the callback.

  This method is not thread-safe.

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

  A thread-safe variant of `call_soon()`. Must be used to schedule callbacks from another thread.

  查看 并发和多线程 章节的文档。

在 3.7 版更改: The context keyword-only parameter was added. See PEP 567 for more details.

注解

Most `asyncio` scheduling functions don't allow passing keyword arguments. To do that, use `functools.partial()`:

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

Using partial objects is usually more convenient than using lambdas, as asyncio can render partial objects better in debug and error messages.

### Scheduling delayed callbacks

Event loop provides mechanisms to schedule callback functions to be called at some point in the future. Event loop uses monotonic clocks to track time.

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

  Schedule callback to be called after the given delay number of seconds (can be either an int or a float).

  An instance of `asyncio.TimerHandle` is returned which can be used to cancel the callback.

  callback will be called exactly once. If two callbacks are scheduled for exactly the same time, the order in which they are called is undefined.

  The optional positional args will be passed to the callback when it is called. If you want the callback to be called with keyword arguments use `functools.partial()`.

  An optional keyword-only context argument allows specifying a custom `contextvars.Context` for the callback to run in. The current context is used when no context is provided.

  在 3.7 版更改: The context keyword-only parameter was added. See PEP 567 for more details.

  在 3.8 版更改: 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)

  Schedule callback to be called at the given absolute timestamp when (an int or a float), using the same time reference as `loop.time()`.

  这个函数的行为与 `call_later()` 相同。

  An instance of `asyncio.TimerHandle` is returned which can be used to cancel the callback.

  在 3.7 版更改: The context keyword-only parameter was added. See PEP 567 for more details.

  在 3.8 版更改: 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`()

  Return the current time, as a `float` value, according to the event loop's internal monotonic clock.

注解

超时（相对 delay 或者绝对 when ）不能超过一天。

参见

`asyncio.sleep()` 函数

### Creating Futures and Tasks

`loop.``create_future`()

  Create an `asyncio.Future` object attached to the event loop.

  This is the preferred way to create Futures in asyncio. This lets third-party event loops provide alternative implementations of the Future object (with better performance or instrumentation).

  3.5.2 新版功能.

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

  Schedule the execution of a 协程. Return a `Task` object.

  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()`.

  在 3.8 版更改: Added the `name` parameter.

`loop.``set_task_factory`(factory)

  Set a task factory that will be used by `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.

### Opening network connections

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)

  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.

  This method will try to establish the connection in the background. When successful, it returns a `(transport, protocol)` pair.

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

  创建的transport是一个实现相关的双向流。

  Other arguments:

  - 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.

    参见

    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。

  - sock，如果给出的话，应该是一个现有的，已经连接的 `socket.socket` 对象，这个对象将被transport使用。如果给出 sock，则 host，port，family，proto，flags 和 local_addr 中的任何一个都不应该指定。

  - 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).

  3.7 新版功能: The ssl_handshake_timeout parameter.

  在 3.6 版更改: The socket option `TCP_NODELAY` is set by default for all TCP connections.

  在 3.5 版更改: Added support for SSL/TLS in `ProactorEventLoop`.

  参见

  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)

  Create a datagram connection.

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

  The socket type will be `SOCK_DGRAM`.

  protocol_factory must be a callable returning a protocol implementation.

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

  Other arguments:

  - 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_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 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`)。

  On Windows, with `ProactorEventLoop`, this method is not supported.

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

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

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

  Create a Unix connection.

  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 新版功能: The ssl_handshake_timeout parameter.

  在 3.7 版更改: The path parameter can now be a path-like object.

### Creating network servers

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)

  Create a TCP server (socket type `SOCK_STREAM`) listening on port of the host address.

  Returns a `Server` object.

  Arguments:

  - 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 set to `True` (the default) causes the created server to start accepting connections immediately. When set to `False`, the user should await on `Server.start_serving()` or `Server.serve_forever()` to make the server to start accepting connections.

  3.7 新版功能: Added ssl_handshake_timeout and start_serving parameters.

  在 3.6 版更改: The socket option `TCP_NODELAY` is set by default for all TCP connections.

  在 3.5 版更改: Added support for SSL/TLS in `ProactorEventLoop`.

  在 3.5.1 版更改: The host parameter can be a sequence of strings.

  参见

  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 新版功能: The ssl_handshake_timeout and start_serving parameters.

  在 3.7 版更改: The path parameter can now be a `Path` object.

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 is (for an SSL connection) the time in seconds to wait for the SSL handshake to complete before aborting the connection. `60.0` seconds if `None` (default).

  Returns a `(transport, protocol)` pair.

  3.7 新版功能: The ssl_handshake_timeout parameter.

  3.5.3 新版功能.

### Transferring files

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

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

  The method uses high-performance `os.sendfile()` if available.

  file must be a regular file object opened in binary mode.

  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 新版功能.

### TLS Upgrade

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: a configured instance of `SSLContext`.
  - server_side pass `True` when a server-side connection is being upgraded (like the one created by `create_server()`).
  - server_hostname: sets or overrides the host name that the target server's certificate will be matched against.
  - 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 新版功能.

### Watching file descriptors

`loop.``add_reader`(fd, callback, *args)

  Start monitoring the fd file descriptor for read availability and invoke callback with the specified arguments once fd is available for reading.

`loop.``remove_reader`(fd)

  Stop monitoring the fd file descriptor for read availability.

`loop.``add_writer`(fd, callback, *args)

  Start monitoring the fd file descriptor for write availability and invoke callback with the specified arguments once fd is available for writing.

  Use `functools.partial()` to pass keyword arguments to callback.

`loop.``remove_writer`(fd)

  Stop monitoring the fd file descriptor for write availability.

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

### Working with socket objects directly

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()`.

  Return the received data as a bytes object.

  sock must be a non-blocking socket.

  在 3.7 版更改: 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.

  Return the number of bytes written to the buffer.

  sock must be a non-blocking socket.

  3.7 新版功能.

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 must be a non-blocking socket.

  在 3.7 版更改: Even though the method was always documented as a coroutine method, before Python 3.7 it returned an `Future`. Since Python 3.7, this is an `async def` method.

coroutine `loop.``sock_connect`(sock, address)

  Connect sock to a remote socket at address.

  Asynchronous version of `socket.connect()`.

  sock must be a non-blocking socket.

  在 3.5.2 版更改: `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.

  参见

  `loop.create_connection()` and `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 must be a non-blocking socket.

  在 3.7 版更改: Even though the method was always documented as a coroutine method, before Python 3.7 it returned a `Future`. Since Python 3.7, this is an `async def` method.

  参见

  `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 must be a regular file object open in binary mode.

  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).

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

  sock must be a non-blocking socket.

  3.7 新版功能.

### DNS

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

  Asynchronous version of `socket.getaddrinfo()`.

coroutine `loop.``getnameinfo`(sockaddr, flags=0)

  Asynchronous version of `socket.getnameinfo()`.

在 3.7 版更改: Both getaddrinfo and getnameinfo methods were always documented to return a coroutine, but prior to Python 3.7 they were, in fact, returning `asyncio.Future` objects. Starting with Python 3.7 both methods are coroutines.

### Working with pipes

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 is a file-like object.

  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 is file-like object.

  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。

注解

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

参见

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

### Unix signals

`loop.``add_signal_handler`(signum, callback, *args)

  Set callback as the handler for the signum signal.

  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` 。

  Use `functools.partial()` to pass keyword arguments to callback.

  Like `signal.signal()`, this function must be invoked in the main thread.

`loop.``remove_signal_handler`(sig)

  Remove the handler for the sig signal.

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

  可用性: Unix。

参见

`signal` 模块。

### Executing code in thread or process pools

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

  Arrange for func to be called in the specified executor.

  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())

  This method returns a `asyncio.Future` object.

  Use `functools.partial()` to pass keyword arguments to func.

  在 3.5.3 版更改: `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 版后已移除: Using an executor that is not an instance of `ThreadPoolExecutor` is deprecated and will trigger an error in Python 3.9.

  executor must be an instance of `concurrent.futures.ThreadPoolExecutor`.

### 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`()

  Return the current exception handler, or `None` if no custom exception handler was set.

  3.5.2 新版功能.

`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 is a `dict` object containing the following keys (new keys may be introduced in future Python versions):

  - '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。

  注解

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

### Enabling debug mode

`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。

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

参见

debug mode of asyncio.

### Running Subprocesses

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.

注解

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)

  Create a subprocess from one or more string arguments specified by args.

  args must be a list of strings represented by:

  - `str`;
  - or `bytes`, encoded to the filesystem encoding.

  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.

  Other parameters:

  - stdin: either a file-like object representing a pipe to be connected to the subprocess's standard input stream using `connect_write_pipe()`, or the `subprocess.PIPE` constant (default). By default a new pipe will be created and connected.

  - stdout: either a file-like object representing the pipe to be connected to the subprocess's standard output stream using `connect_read_pipe()`, or the `subprocess.PIPE` constant (default). By default a new pipe will be created and connected.

  - stderr: either a file-like object representing the pipe to be connected to the subprocess's standard error stream using `connect_read_pipe()`, or one of `subprocess.PIPE` (default) or `subprocess.STDOUT` constants.

    By default a new pipe will be created and connected. When `subprocess.STDOUT` is specified, the subprocess' standard error stream will be connected to the same pipe as the standard output stream.

  - All other keyword arguments are passed to `subprocess.Popen` without interpretation, except for bufsize, universal_newlines and shell, which should not be specified at all.

  See the constructor of the `subprocess.Popen` class for documentation on other arguments.

  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.

  This is similar to the standard library `subprocess.Popen` class called with `shell=True`.

  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.

注解

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.

## Callback Handles

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 新版功能.

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 新版功能.

## 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 objects are asynchronous context managers. When used in an `async with` statement, it's guaranteed that the Server object is closed and not accepting new connections when the `async with` statement is completed:

      srv = await loop.create_server(...)

      async with srv:
          # some code

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

  在 3.7 版更改: Server object is an asynchronous context manager since Python 3.7.

  `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 新版功能.

  coroutine `start_serving`()

    Start accepting connections.

    This method is idempotent, so it can be called when the server is already being serving.

    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 新版功能.

  coroutine `serve_forever`()

    Start accepting connections until the coroutine is cancelled. Cancellation of `serve_forever` task causes the server to be closed.

    This method can be called if the server is already accepting connections. Only one `serve_forever` task can exist per one Server object.

    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 新版功能.

  `is_serving`()

    Return `True` if the server is accepting new connections.

    3.7 新版功能.

  coroutine `wait_closed`()

    等待 `close()` 方法执行完毕。

  `sockets`

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

    在 3.7 版更改: 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.

## Event Loop Implementations

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

By default asyncio is configured to use `SelectorEventLoop` on all platforms.

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)

  Availability: Unix, Windows.

class `asyncio.``ProactorEventLoop`

  An event loop for Windows that uses "I/O Completion Ports" (IOCP).

  Availability: Windows.

  参见

  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()

参见

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()

参见

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

Wait until a file descriptor received some data using the `loop.add_reader()` method and then close the event loop:

    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()

参见

- 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):
        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))

        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())