# 18.5.1. Base Event Loop

Source code: Lib/asyncio/events.py

The event loop is the central execution device provided by `asyncio`. It provides multiple facilities, including:

- Registering, executing and cancelling delayed calls (timeouts).

- Creating client and server transports for various kinds of communication.

- Launching subprocesses and the associated transports for communication with an external program.

- Delegating costly function calls to a pool of threads.

class `asyncio.``BaseEventLoop`

  This class is an implementation detail. It is a subclass of `AbstractEventLoop` and may be a base class of concrete event loop implementations found in `asyncio`. It should not be used directly; use `AbstractEventLoop` instead. `BaseEventLoop` should not be subclassed by third-party code; the internal interface is not stable.

class `asyncio.``AbstractEventLoop`

  Abstract base class of event loops.

  This class is not thread safe.

## 18.5.1.1. Run an event loop

`AbstractEventLoop.``run_forever`()

  Run until `stop()` is called. If `stop()` is called before `run_forever()` is called, this polls the I/O selector once with a timeout of zero, runs all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exits. If `stop()` is called while `run_forever()` is running, this will run the current batch of callbacks and then exit. Note that callbacks scheduled by callbacks will not run in that case; they will run the next time `run_forever()` is called.

  3.5.1 版更變.

`AbstractEventLoop.``run_until_complete`(future)

  Run until the `Future` is done.

  If the argument is a coroutine object, it is wrapped by `ensure_future()`.

  Return the Future’s result, or raise its exception.

`AbstractEventLoop.``is_running`()

  Returns running status of event loop.

`AbstractEventLoop.``stop`()

  Stop running the event loop.

  This causes `run_forever()` to exit at the next suitable opportunity (see there for more details).

  3.5.1 版更變.

`AbstractEventLoop.``is_closed`()

  Returns `True` if the event loop was closed.

  3.4.2 版新加入.

`AbstractEventLoop.``close`()

  Close the event loop. The loop must not be running. Pending callbacks will be lost.

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

  This is idempotent and irreversible. No other methods should be called after this one.

coroutine `AbstractEventLoop.``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 whenever a new asynchronous generator is iterated. Should be used to finalize all scheduled asynchronous generators reliably. Example:

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

  3.6 版新加入.

## 18.5.1.2. Calls

Most `asyncio` functions don’t accept keywords. If you want to pass keywords to your callback, use `functools.partial()`. For example, `loop.call_soon(functools.partial(print, "Hello", flush=True))` will call `print("Hello", flush=True)`.

備註

`functools.partial()` is better than `lambda` functions, because `asyncio` can inspect `functools.partial()` object to display parameters in debug mode, whereas `lambda` functions have a poor representation.

`AbstractEventLoop.``call_soon`(callback, *args)

  Arrange for a callback to be called as soon as possible. The callback is called after `call_soon()` returns, when control returns to the event loop.

  This operates as a FIFO queue, callbacks are called in the order in which they are registered. Each callback will be called exactly once.

  Any positional arguments after the callback will be passed to the callback when it is called.

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

  Use functools.partial to pass keywords to the callback.

`AbstractEventLoop.``call_soon_threadsafe`(callback, *args)

  Like `call_soon()`, but thread safe.

  参见 concurrency and multithreading 部分的文档。

## 18.5.1.3. Delayed calls

The event loop has its own internal clock for computing timeouts. Which clock is used depends on the (platform-specific) event loop implementation; ideally it is a monotonic clock. This will generally be a different clock than `time.time()`.

備註

Timeouts (relative delay or absolute when) should not exceed one day.

`AbstractEventLoop.``call_later`(delay, callback, *args)

  Arrange for the callback to be called after the given delay seconds (either an int or float).

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

  callback will be called exactly once per call to `call_later()`. If two callbacks are scheduled for exactly the same time, it is undefined which will be called first.

  The optional positional args will be passed to the callback when it is called. If you want the callback to be called with some named arguments, use a closure or `functools.partial()`.

  Use functools.partial to pass keywords to the callback.

`AbstractEventLoop.``call_at`(when, callback, *args)

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

  本方法的行为和 `call_later()` 方法相同。

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

  Use functools.partial to pass keywords to the callback.

`AbstractEventLoop.``time`()

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

也參考

`asyncio.sleep()` 函数。

## 18.5.1.4. Futures

`AbstractEventLoop.``create_future`()

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

  This is a preferred way to create futures in asyncio, as event loop implementations can provide alternative implementations of the Future class (with better performance or instrumentation).

  3.5.2 版新加入.

## 18.5.1.5. Tasks

`AbstractEventLoop.``create_task`(coro)

  Schedule the execution of a coroutine object: wrap it in a future. Return a `Task` object.

  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。

  This method was added in Python 3.4.2. Use the `async()` function to support also older Python versions.

  3.4.2 版新加入.

`AbstractEventLoop.``set_task_factory`(factory)

  Set a task factory that will be used by `AbstractEventLoop.create_task()`.

  If factory is `None` the default task factory will be set.

  If factory is a callable, it should have a signature matching `(loop, coro)`, where loop will be a reference to the active event loop, coro will be a coroutine object. The callable must return an `asyncio.Future` compatible object.

  3.4.4 版新加入.

`AbstractEventLoop.``get_task_factory`()

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

  3.4.4 版新加入.

## 18.5.1.6. Creating connections

coroutine `AbstractEventLoop.``create_connection`(protocol_factory, host=None, port=None, *, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None)

  Create a streaming transport connection to a given Internet host and port: socket family `AF_INET` or `AF_INET6` depending on host (or family if specified), socket type `SOCK_STREAM`. protocol_factory must be a callable returning a protocol instance.

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

  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 to represent it.

  2. protocol_factory is called without arguments and must return a protocol instance.

  3. The protocol instance is tied to the transport, and its `connection_made()` method is called.

  4. The coroutine returns successfully with the `(transport, protocol)` pair.

  被创建的传输对象是一个实现相关的双向流。

  備註

  protocol_factory can be any kind of callable, not necessarily a class. For example, if you want to use a pre-created protocol instance, you can pass `lambda: my_protocol`.

  Options that change how the connection is created:

  - 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 context with some unspecified default settings is used.

    也參考

    SSL/TLS security considerations

  - server_hostname, is only for use together with ssl, and sets or overrides the hostname that the target server’s certificate will be matched against. 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 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。

  - sock，如果指定的话，其应该是一个已经存在，并且已经处于连接状态的 `socket.socket` 对象，其会被传输对象使用。如果指定了 sock ，那么 host, port, family, proto, flags 和 local_addr 就都不应该被指定了。

  - 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.

  3.5 版更變: On Windows with `ProactorEventLoop`, SSL/TLS is now supported.

  也參考

  The `open_connection()` function can be used to get a pair of (`StreamReader`, `StreamWriter`) instead of a protocol.

coroutine `AbstractEventLoop.``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 indentical 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。

  Create datagram connection: socket family `AF_INET` or `AF_INET6` depending on host (or family if specified), socket type `SOCK_DGRAM`. protocol_factory must be a callable returning a protocol instance.

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

  Options changing how the connection is created:

  - 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 UNIX’s. 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。

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

  3.6.10 版更變: The reuse_address parameter is no longer supporter due to security concerns

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

  Create UNIX connection: socket family `AF_UNIX`, socket type `SOCK_STREAM`. The `AF_UNIX` socket family is used to communicate between processes on the same machine efficiently.

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

  path is the name of a UNIX domain socket, and is required unless a sock parameter is specified. Abstract UNIX sockets, `str`, and `bytes` paths are supported.

  See the `AbstractEventLoop.create_connection()` method for parameters.

  Availability: UNIX.

## 18.5.1.7. Creating listening connections

coroutine `AbstractEventLoop.``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)

  Create a TCP server (socket type `SOCK_STREAM`) bound to host and port.

  Return a `Server` object, its `sockets` attribute contains created sockets. Use the `Server.close()` method to stop the server: close listening sockets.

  参数：

  - The host parameter can be a string, in that case the TCP server is bound to host and port. The host parameter can also be a sequence of strings and in that case the TCP server is bound to all hosts of 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 it will be determined from host (defaults to `socket.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 should be omitted (must be `None`).

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

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

  - 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。

  This method is a coroutine.

  3.5 版更變: On Windows with `ProactorEventLoop`, SSL/TLS is now supported.

  也參考

  The function `start_server()` creates a (`StreamReader`, `StreamWriter`) pair and calls back a function with this pair.

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

coroutine `AbstractEventLoop.``create_unix_server`(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None)

  Similar to `AbstractEventLoop.create_server()`, but specific to the socket family `AF_UNIX`.

  This method is a coroutine.

  Availability: UNIX.

coroutine `BaseEventLoop.``connect_accepted_socket`(protocol_factory, sock, *, ssl=None)

  Handle an accepted connection.

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

  参数：

  - sock is a preexisting socket object returned from an `accept` call.

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

  This method is a coroutine. When completed, the coroutine returns a `(transport, protocol)` pair.

  3.5.3 版新加入.

## 18.5.1.8. Watch file descriptors

On Windows with `SelectorEventLoop`, only socket handles are supported (ex: pipe file descriptors are not supported).

On Windows with `ProactorEventLoop`, these methods are not supported.

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

  Start watching the file descriptor for read availability and then call the callback with specified arguments.

  Use functools.partial to pass keywords to the callback.

`AbstractEventLoop.``remove_reader`(fd)

  Stop watching the file descriptor for read availability.

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

  Start watching the file descriptor for write availability and then call the callback with specified arguments.

  Use functools.partial to pass keywords to the callback.

`AbstractEventLoop.``remove_writer`(fd)

  Stop watching the file descriptor for write availability.

The watch a file descriptor for read events example uses the low-level `AbstractEventLoop.add_reader()` method to register the file descriptor of a socket.

## 18.5.1.9. Low-level socket operations

coroutine `AbstractEventLoop.``sock_recv`(sock, nbytes)

  Receive data from the socket. Modeled after blocking `socket.socket.recv()` method.

  The return value is a bytes object representing the data received. The maximum amount of data to be received at once is specified by nbytes.

  With `SelectorEventLoop` event loop, the socket sock must be non-blocking.

  This method is a coroutine.

coroutine `AbstractEventLoop.``sock_sendall`(sock, data)

  Send data to the socket. Modeled after blocking `socket.socket.sendall()` method.

  The socket must be connected to a remote socket. This method continues to send data from data until either all data has been sent or an error occurs. `None` is returned on success. On error, an exception is raised, and there is no way to determine how much data, if any, was successfully processed by the receiving end of the connection.

  With `SelectorEventLoop` event loop, the socket sock must be non-blocking.

  This method is a coroutine.

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

  Connect to a remote socket at address. Modeled after blocking `socket.socket.connect()` method.

  With `SelectorEventLoop` event loop, the socket sock must be non-blocking.

  This method is a coroutine.

  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, `AbstractEventLoop.getaddrinfo()` will be used to resolve the address.

  也參考

  `AbstractEventLoop.create_connection()` and `asyncio.open_connection()`.

coroutine `AbstractEventLoop.``sock_accept`(sock)

  Accept a connection. Modeled after blocking `socket.socket.accept()`.

  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。

  The socket sock must be non-blocking.

  This method is a coroutine.

  也參考

  `AbstractEventLoop.create_server()` and `start_server()`.

## 18.5.1.10. Resolve host name

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

  This method is a coroutine, similar to `socket.getaddrinfo()` function but non-blocking.

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

  This method is a coroutine, similar to `socket.getnameinfo()` function but non-blocking.

## 18.5.1.11. Connect pipes

On Windows with `SelectorEventLoop`, these methods are not supported. Use `ProactorEventLoop` to support pipes on Windows.

coroutine `AbstractEventLoop.``connect_read_pipe`(protocol_factory, pipe)

  Register read pipe in eventloop.

  protocol_factory should instantiate object with `Protocol` interface. pipe is a file-like object. Return pair `(transport, protocol)`, where transport supports the `ReadTransport` interface.

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

  This method is a coroutine.

coroutine `AbstractEventLoop.``connect_write_pipe`(protocol_factory, pipe)

  Register write pipe in eventloop.

  protocol_factory should instantiate object with `BaseProtocol` interface. pipe is file-like object. Return pair `(transport, protocol)`, where transport supports `WriteTransport` interface.

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

  This method is a coroutine.

也參考

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

## 18.5.1.12. UNIX signals

Availability: UNIX only.

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

  Add a handler for a signal.

  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 keywords to the callback.

`AbstractEventLoop.``remove_signal_handler`(sig)

  Remove a handler for a signal.

  Return `True` if a signal handler was removed, `False` if not.

也參考

`signal` 模块。

## 18.5.1.13. Executor

Call a function in an `Executor` (pool of threads or pool of processes). By default, an event loop uses a thread pool executor (`ThreadPoolExecutor`).

coroutine `AbstractEventLoop.``run_in_executor`(executor, func, *args)

  Arrange for a func to be called in the specified executor.

  The executor argument should be an `Executor` instance. The default executor is used if executor is `None`.

  Use functools.partial to pass keywords to the *func*.

  This method is a coroutine.

  3.5.3 版更變: `BaseEventLoop.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.

`AbstractEventLoop.``set_default_executor`(executor)

  Set the default executor used by `run_in_executor()`.

## 18.5.1.14. 错误处理API

允许自定义事件循环中如何去处理异常。

`AbstractEventLoop.``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.

  If handler is a callable object, it should have a matching signature to `(loop, context)`, where `loop` will be a reference to the active event loop, `context` will be a `dict` object (see `call_exception_handler()` documentation for details about context).

`AbstractEventLoop.``get_exception_handler`()

  Return the exception handler, or `None` if the default one is in use.

  3.5.2 版新加入.

`AbstractEventLoop.``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, and can be called by a custom exception handler that wants to defer to the default behavior.

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

`AbstractEventLoop.``call_exception_handler`(context)

  调用当前事件循环的异常处理器。

  context is a `dict` object containing the following keys (new keys may be introduced later):

  - 『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

  Note: this method should not be overloaded in subclassed event loops. For any custom exception handling, use `set_exception_handler()` method.

## 18.5.1.15. Debug mode

`AbstractEventLoop.``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` 。

  3.4.2 版新加入.

`AbstractEventLoop.``set_debug`(enabled: bool)

  设置事件循环的调试模式。

  3.4.2 版新加入.

也參考

debug mode of asyncio.

## 18.5.1.16. Server

class `asyncio.``Server`

  Server listening on sockets.

  Object created by the `AbstractEventLoop.create_server()` method and the `start_server()` function. Don’t instantiate the class directly.

  `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。

  coroutine `wait_closed`()

    等待 `close()` 方法执行完毕。

    This method is a coroutine.

  `sockets`

    List of `socket.socket` objects the server is listening to, or `None` if the server is closed.

## 18.5.1.17. Handle

class `asyncio.``Handle`

  A callback wrapper object returned by `AbstractEventLoop.call_soon()`, `AbstractEventLoop.call_soon_threadsafe()`, `AbstractEventLoop.call_later()`, and `AbstractEventLoop.call_at()`.

  `cancel`()

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

## 18.5.1.18. Event loop examples

### 18.5.1.18.1. call_soon() 的 Hello World 示例。

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

    import asyncio

    def hello_world(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()
    loop.run_forever()
    loop.close()

也參考

The Hello World coroutine example uses a coroutine.

### 18.5.1.18.2. 使用 call_later() 来展示当前的日期

Example of callback displaying the current date every second. The callback uses the `AbstractEventLoop.call_later()` method to reschedule itself during 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()
    loop.run_forever()
    loop.close()

也參考

The coroutine displaying the current date example uses a coroutine.

### 18.5.1.18.3. 监控一个文件描述符的读事件

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

    import asyncio
    try:
        from socket import socketpair
    except ImportError:
        from asyncio.windows_utils 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())

    # Run the event loop
    loop.run_forever()

    # We are done, close sockets and the event loop
    rsock.close()
    wsock.close()
    loop.close()

也參考

The register an open socket to wait for data using a protocol example uses a low-level protocol created by the `AbstractEventLoop.create_connection()` method.

The register an open socket to wait for data using streams example uses high-level streams created by the `open_connection()` function in a coroutine.

### 18.5.1.18.4. 为SIGINT和SIGTERM设置信号处理器

Register handlers for signals `SIGINT` and `SIGTERM` using the `AbstractEventLoop.add_signal_handler()` method:

    import asyncio
    import functools
    import os
    import signal

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

    loop = asyncio.get_event_loop()
    for signame in ('SIGINT', 'SIGTERM'):
        loop.add_signal_handler(getattr(signal, signame),
                                functools.partial(ask_exit, signame))

    print("Event loop running forever, press Ctrl+C to interrupt.")
    print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
    try:
        loop.run_forever()
    finally:
        loop.close()

This example only works on UNIX.