18.4. selectors — 高级 I/O 复用 ¶

18.4.1. 介绍 ¶

This module allows high-level and efficient I/O multiplexing, built upon the select module primitives. Users are encouraged to use this module instead, unless they want precise control over the OS-level primitives used.

It defines a BaseSelector abstract base class, along with several concrete implementations ( KqueueSelector , EpollSelector …), that can be used to wait for I/O readiness notification on multiple file objects. In the following, “file object” refers to any object with a fileno() method, or a raw file descriptor. See 文件对象 .

DefaultSelector is an alias to the most efficient implementation available on the current platform: this should be the default choice for most users.

18.4.2. 类 ¶

Classes hierarchy:

BaseSelector
+-- SelectSelector
+-- PollSelector
+-- EpollSelector
+-- DevpollSelector
+-- KqueueSelector
								

In the following, events is a bitwise mask indicating which I/O events should be waited for on a given file object. It can be a combination of the modules constants below:

常量	含义
EVENT_READ	Available for read
EVENT_WRITE	Available for write

class selectors. SelectorKey ¶

A SelectorKey 是 namedtuple used to associate a file object to its underlying file descriptor, selected event mask and attached data. It is returned by several BaseSelector 方法。

fileobj ¶

File object registered.

fd ¶

Underlying file descriptor.

events ¶

Events that must be waited for on this file object.

data ¶

Optional opaque data associated to this file object: for example, this could be used to store a per-client session ID.

class selectors. BaseSelector ¶

A BaseSelector is used to wait for I/O event readiness on multiple file objects. It supports file stream registration, unregistration, and a method to wait for I/O events on those streams, with an optional timeout. It’s an abstract base class, so cannot be instantiated. Use DefaultSelector instead, or one of SelectSelector , KqueueSelector etc. if you want to specifically use an implementation, and your platform supports it. BaseSelector and its concrete implementations support the 上下文管理器 协议。

abstractmethod register ( fileobj , events , data=None ) ¶

Register a file object for selection, monitoring it for I/O events.

fileobj is the file object to monitor. It may either be an integer file descriptor or an object with a fileno() 方法。 events is a bitwise mask of events to monitor. data is an opaque object.

This returns a new SelectorKey instance, or raises a ValueError in case of invalid event mask or file descriptor, or KeyError if the file object is already registered.

abstractmethod unregister ( fileobj ) ¶

Unregister a file object from selection, removing it from monitoring. A file object shall be unregistered prior to being closed.

fileobj must be a file object previously registered.

This returns the associated SelectorKey instance, or raises a KeyError if fileobj is not registered. It will raise ValueError if fileobj is invalid (e.g. it has no fileno() method or its fileno() method has an invalid return value).

modify ( fileobj , events , data=None ) ¶

Change a registered file object’s monitored events or attached data.

这相当于 BaseSelector.unregister(fileobj)() followed by BaseSelector.register(fileobj, events, data)() , except that it can be implemented more efficiently.

This returns a new SelectorKey instance, or raises a ValueError in case of invalid event mask or file descriptor, or KeyError if the file object is not registered.

abstractmethod select ( timeout=None ) ¶

Wait until some registered file objects become ready, or the timeout expires.

若 timeout > 0 , this specifies the maximum wait time, in seconds. If timeout <= 0 , the call won’t block, and will report the currently ready file objects. If timeout is None , the call will block until a monitored file object becomes ready.

This returns a list of (key, events) tuples, one for each ready file object.

key 是 SelectorKey instance corresponding to a ready file object. events is a bitmask of events ready on this file object.

注意

This method can return before any file object becomes ready or the timeout has elapsed if the current process receives a signal: in this case, an empty list will be returned.

3.5 版改变： The selector is now retried with a recomputed timeout when interrupted by a signal if the signal handler did not raise an exception (see PEP 475 for the rationale), instead of returning an empty list of events before the timeout.

close ( ) ¶

Close the selector.

This must be called to make sure that any underlying resource is freed. The selector shall not be used once it has been closed.

get_key ( fileobj ) ¶

Return the key associated with a registered file object.

This returns the SelectorKey instance associated to this file object, or raises KeyError if the file object is not registered.

abstractmethod get_map ( ) ¶

Return a mapping of file objects to selector keys.

This returns a Mapping instance mapping registered file objects to their associated SelectorKey 实例。

class selectors. DefaultSelector ¶

The default selector class, using the most efficient implementation available on the current platform. This should be the default choice for most users.

class selectors. SelectSelector ¶

select.select() -based selector.

class selectors. PollSelector ¶

select.poll() -based selector.

class selectors. EpollSelector ¶

select.epoll() -based selector.

fileno ( ) ¶

This returns the file descriptor used by the underlying select.epoll() 对象。

class selectors. DevpollSelector ¶

select.devpoll() -based selector.

fileno ( ) ¶

This returns the file descriptor used by the underlying select.devpoll() 对象。

3.5 版新增。

class selectors. KqueueSelector ¶

select.kqueue() -based selector.

fileno ( ) ¶

This returns the file descriptor used by the underlying select.kqueue() 对象。

18.4.3. 范例 ¶

Here is a simple echo server implementation:

import selectors
import socket
sel = selectors.DefaultSelector()
def accept(sock, mask):
    conn, addr = sock.accept()  # Should be ready
    print('accepted', conn, 'from', addr)
    conn.setblocking(False)
    sel.register(conn, selectors.EVENT_READ, read)
def read(conn, mask):
    data = conn.recv(1000)  # Should be ready
    if data:
        print('echoing', repr(data), 'to', conn)
        conn.send(data)  # Hope it won't block
    else:
        print('closing', conn)
        sel.unregister(conn)
        conn.close()
sock = socket.socket()
sock.bind(('localhost', 1234))
sock.listen(100)
sock.setblocking(False)
sel.register(sock, selectors.EVENT_READ, accept)
while True:
    events = sel.select()
    for key, mask in events:
        callback = key.data
        callback(key.fileobj, mask)