Curio - The coroutine concurrency library.
Python Makefile
Latest commit 20ae7b8 Jan 17, 2017 @dabeaz Some doc updates


curio - concurrent I/O

Curio is a library for performing concurrent I/O and common system programming tasks such as launching subprocesses and farming work out to thread and process pools. It uses Python coroutines and the explicit async/await syntax introduced in Python 3.5. Its programming model is based on cooperative multitasking and existing programming abstractions such as threads, sockets, files, subprocesses, locks, and queues. You'll find it to be small and fast.

Important Disclaimer

Curio is experimental software that currently only works on POSIX systems (OS X, Linux, etc.). It is a work in progress and it may change at any time. Although curio can be installed via pip, the version uploaded on PyPI is only updated occasionally. You'll probably get better results using the version cloned from github. Of course, your mileage might vary.


The version 0.4 release of curio cleans up a lot of APIs and makes a wide variety of performance improvements. If you were using version 0.1, a lot of stuff is probably going to break. Sorry about that. However, curio is still in its infancy. APIs might change—hopefully for the better though.

Quick install

pip install git+

An Example

Here is a simple TCP echo server implemented using sockets and curio:


from curio import run, spawn
from curio.socket import *

async def echo_server(address):
    sock = socket(AF_INET, SOCK_STREAM)
    sock.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
    print('Server listening at', address)
    async with sock:
        while True:
            client, addr = await sock.accept()
            await spawn(echo_client(client, addr))

async def echo_client(client, addr):
    print('Connection from', addr)
    async with client:
         while True:
             data = await client.recv(100000)
             if not data:
             await client.sendall(data)
    print('Connection closed')

if __name__ == '__main__':

If you have programmed with threads, you find that curio looks similar. You'll also find that the above server can handle thousands of simultaneous client connections even though no threads are being used under the covers.

Of course, if you prefer something a little higher level, you can have curio take care of the fiddly bits related to setting up the server portion of the code:


from curio import run, tcp_server

async def echo_client(client, addr):
    print('Connection from', addr)
    while True:
        data = await client.recv(100000)
        if not data:
        await client.sendall(data)
    print('Connection closed')

if __name__ == '__main__':
    run(tcp_server('', 25000, echo_client))

This is only a small sample of what's possible. Read the official documentation for more in-depth coverage. The tutorial is a good starting point. The howto describes how to carry out various tasks.

Additional Features

Curio provides additional support for SSL connections, synchronization primitives (events, locks, recursive locks, semaphores, and condition variables), queues, Unix signals, subprocesses, as well as running tasks in threads and processes. The task model fully supports cancellation, timeouts, monitoring, and other features critical to writing reliable code.

Projects Using Curio

Please feel free to add your project here.

Talks Related to Curio

The Big Question: Why?

Python already has a variety of libraries for async and event driven I/O. So, why create yet another library? There is no simple answer to that question, but here are a few of the motivations for creating curio.

  • Python 3 has evolved considerably as a programming language and has adopted many new language features that are well-suited to cleanly writing a library like this. For example, improved support for non-blocking I/O, support for delegation to subgenerators (yield from) and the introduction of explicit async and await syntax in Python 3.5. Curio takes full advantage of these features and is not encumbered by issues of backwards compatibility with legacy Python code written 15 years ago.
  • Existing I/O libraries are mainly built on event-loops, callback functions, futures, and various abstractions that predate Python's proper support for coroutines. As a result, they are either overly complicated or dependent on esoteric magic involving C extensions, monkeypatching, or reimplementing half of the TCP flow-control protocol. Curio is a ground-up implementation that takes a different approach to the problem while relying upon known programming techniques involving sockets and files. If you have previously written synchronous code using processes or threads, curio will feel familiar. That is by design.
  • Simplicity is an important part of writing reliable systems software. When your code fails, it helps to be able to debug it--possibly down to the level of individual calls to the operating system if necessary. Simplicity matters a lot. Simple code also tends to run faster. The implementation of Curio aims to be simple. The API for using Curio aims to be intuitive.
  • It's fun.

Questions and Answers

Q: Is curio implemented using the asyncio module?

A: No. Curio is a standalone library. Although the core of the library uses the same basic machinery as asyncio to poll for I/O events, the handling of those events is carried out in a completely different manner.

Q: Is curio meant to be a clone of asyncio?

A: No. Although curio provides a significant amount of overlapping functionality, the API is different and smaller. Compatibility with other libaries is not a goal.

Q: How many tasks can be created?

A: Each task involves an instance of a Task class that encapsulates a generator. No threads are used. As such, you're really only limited by the memory of your machine--potentially you could have hundreds of thousands of tasks. The I/O functionality in curio is implemented using the built-in selectors module. Thus, the number of open sockets allowed would be subject to the limits of that library combined with any per-user limits imposed by the operating system.

Q: Can curio interoperate with other event loops?

A: At this time, no. However, curio is a young project. It's something that might be added later.

Q: How fast is curio?

A: In rough benchmarking of the simple echo server shown here, Curio runs between 75-150% faster than comparable code using coroutines in asyncio, 5-40% faster than the same coroutines running on uvloop (an alternative event-loop for asyncio), and at about the same speed as gevent. This is on OS-X so your mileage might vary. Curio is not as fast as servers that utilize threads, low-level callback-based event handling (e.g., low-level protocols in asyncio), or direct coding in assembly language. However, those approaches also don't involve coroutines (which is the whole point of Curio). See the examples/benchmark directory of the distribution for various testing programs.

Q: Is curio going to evolve into a framework?

A: No. The current goal is merely to provide a small, simple library for performing concurrent I/O, task synchronization, and common systems operations involving interprocess communication and subprocesses. It is not anticipated that curio itself would evolve into a framework for implementing application level protocols such as HTTP. However, it might serve as a foundation for other packages that want to provide that kind of functionality.

Q: What are future plans?

A: Future work on curio will primarily focus on features related to performance, debugging, diagnostics, and reliability. A main goal is to provide a robust environment for running and controlling concurrent tasks.

Q: Is there a Curio sticker?

A: No.

Q: How big is curio?

A: The complete library currently consists of fewer than 2500 lines of source statements. This does not include blank lines and comments.

Q: Can I contribute?

A: Absolutely. Please use the Github page at as the primary point of discussion concerning pull requests, bugs, and feature requests.


Read the official docs here:

Discussion Forum

A discussion forum for Curio is available at Please go there to ask questions and find out whats happening with the project.


  • David Beazley
  • Brett Cannon
  • Nathaniel Smith
  • Alexander Zhukov


Curio was created by David Beazley (@dabeaz).

It is a young project. All contributions welcome.