17.9. selectors – High-level I/O multiplexing

New in version 3.4.

17.9.1. Introduction

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 file object.

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

Note

The type of file objects supported depends on the platform: on Windows, sockets are supported, but not pipes, whereas on Unix, both are supported (some other types may be supported as well, such as fifos or special file devices).

See also

select

Low-level I/O multiplexing module.

17.9.2. Classes

Classes hierarchy:

BaseSelector
+-- SelectSelector
+-- PollSelector
+-- EpollSelector
+-- 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 constants below:

Constant	Meaning
EVENT_READ	Available for read
EVENT_WRITE	Available for write

class selectors.SelectorKey

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

fileobj

File object registered.

fd

Underlying file descriptor.

events

Events that must be waited for this file object.

data

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

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 context manager protocol.

register(fileobj, events, data=None)

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

fileobj is the file object to monitor. 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.

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 the file object is not registered.

modify(fileobj, events, data=None)

Change a registered file object monitored events or attached data.

This is equivalent to 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.

select(timeout=None)

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

If 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) tuple, one for each ready file object.

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

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 to a registered file object.

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

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() object.

class selectors.KqueueSelector

select.kqueue()-based selector.

fileno()

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

Examples of selector usage:

>>> import selectors
>>> import socket
>>>
>>> s = selectors.DefaultSelector()
>>> r, w = socket.socketpair()
>>>
>>> s.register(r, selectors.EVENT_READ)
SelectorKey(fileobj=<socket.socket fd=4, family=1, type=1, proto=0>, fd=4, events=1, data=None)
>>> s.register(w, selectors.EVENT_WRITE)
SelectorKey(fileobj=<socket.socket fd=5, family=1, type=1, proto=0>, fd=5, events=2, data=None)
>>>
>>> print(s.select())
[(SelectorKey(fileobj=<socket.socket fd=5, family=1, type=1, proto=0>, fd=5, events=2, data=None), 2)]
>>>
>>> for key, events in s.select():
...     if events & selectors.EVENT_WRITE:
...         key.fileobj.send(b'spam')
...
4
>>> for key, events in s.select():
...     if events & selectors.EVENT_READ:
...         print(key.fileobj.recv(1024))
...
b'spam'
>>> s.close()