Skip to main content

Back-ported, durable, and portable selectors

Project description


Backported, durable, and portable selectors designed to replace the standard library selectors module.


  • Support for all major platforms. (Linux, Mac OS, Windows)

  • Support for Python 2.6 or later and Jython.

  • Support many different selectors
    • select.kqueue (BSD, Mac OS)

    • select.devpoll (Solaris)

    • select.epoll (Linux 2.5.44+)

    • select.poll (Linux, Mac OS)

    • - (Linux, Mac OS, Windows)

  • Support for PEP 475 (Retries system calls on interrupt)

  • Support for modules which monkey-patch the standard library after import (like greenlet, gevent)

  • Support for systems which define a selector being available but don’t actually implement it. ()


This module was originally written by me for the urllib3 project (history in PR #1001) but it was decided that it would be beneficial for everyone to have access to this work.

All the additional features that selectors2 provides are real-world problems that have occurred and been reported during the lifetime of its maintenance and use within urllib3.

If this work is useful to you, feel free to say thanks, takes only a little time and really brightens my day! :cake:

Can this module be used in place of selectors?

Yes! This module is a 1-to-1 drop-in replacement for selectors and provides all selector types that would be available in selectors including DevpollSelector, KqueueSelector, EpollSelector, PollSelector, and SelectSelector.

What is different between selectors2 and selectors34?

This module is similar to selectors34 in that it supports Python 2.6 - 3.3 but differs in that this module also implements PEP 475 for the backported selectors. This allows similar behaviour between Python 3.5+ selectors and selectors from before PEP 475. In selectors34, an interrupted system call would result in an incorrect return of no events, which for some use cases is not an acceptable behavior.

I will also add here that selectors2 also makes large improvements on the test suite surrounding it providing 100% test coverage for each selector. The test suite is also more robust and tests durability of the selectors in many different situations that aren’t tested in selectors34.

What types of objects are supported?

At this current time selectors2 only support the SelectSelector for Windows which cannot select on non-socket objects. On Linux and Mac OS, both sockets and pipes are supported (some other types may be supported as well, such as fifos or special file devices).

What if I have to support a platform without

There are a few platforms that don’t have a selector available, notably Google AppEngine. When running on those platforms any call to DefaultSelector() will raise a RuntimeError explaining that there are no selectors available.


This module is dual-licensed under MIT and PSF License.


$ python -m pip install selectors2


import sys
import selectors2 as selectors

# Use DefaultSelector, it picks the best
# selector available for your platform! :)
s = selectors.DefaultSelector()

import socket

# We're going to use Google as an example.
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("", 80))

# Register the file to be watched for write availibility.
s.register(sock, selectors.EVENT_WRITE)

# Give a timeout in seconds or no
# timeout to block until an event happens.
events =

# Loop over all events that happened.
for key, event in events:
    if event & selectors.EVENT_WRITE:
        key.fileobj.send(b'HEAD / HTTP/1.1\r\n\r\n')

# Change what event you're waiting for.
s.modify(sock, selectors.EVENT_READ)

# Timeout of None let's the selector wait as long as it needs to.
events =
for key, event in events:
    if event & selectors.EVENT_READ:
        data = key.fileobj.recv(4096)

# Stop watching the socket.


Release 2.0.2 (July 21, 2020)

  • [BUGFIX] Added support for long integers in Python 2.x.

Release 2.0.1 (August 17, 2017)

  • [BUGFIX] Timeouts would not be properly recalculated after receiving an EINTR error.

Release 2.0.0 (May 30, 2017)

  • [FEATURE] Add support for Jython with JythonSelectSelector.

  • [FEATURE] Add support for /dev/devpoll with DevpollSelector.

  • [CHANGE] Raises a RuntimeError instead of ValueError if there is no selector available.

  • [CHANGE] No longer wraps exceptions in SelectorError, raises original exception including in timeout situations.

  • [BUGFIX] Detect defects in a system that defines a selector but does not implement it.

  • [BUGFIX] Can now detect a change in the select module after import such as when gevent.monkey.monkey_patch() is called before importing selectors2.

Release 1.1.1 (February 6, 2017)

  • [BUGFIX] Platforms that define select.kqueue would not have KqueueSelector as the DefaultSelector.

Release 1.1.0 (January 17, 2017)

  • [FEATURE] Make system calls faster for Python versions that support PEP 475.

  • [FEATURE] Wheels are now universal.

Release 1.0.0 (November 3, 2016)

  • Initial implementation of selectors2.

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

selectors2-2.0.2.tar.gz (18.6 kB view hashes)

Uploaded source

Built Distribution

selectors2-2.0.2-py2.py3-none-any.whl (11.3 kB view hashes)

Uploaded py2 py3

Supported by

AWS AWS Cloud computing and Security Sponsor Datadog Datadog Monitoring Fastly Fastly CDN Google Google Download Analytics Microsoft Microsoft PSF Sponsor Pingdom Pingdom Monitoring Sentry Sentry Error logging StatusPage StatusPage Status page