Curio - A Tutorial Introduction

Curio is a modern library for performing reliable concurrent I/O using Python coroutines and the explicit async/await syntax introduced in Python 3.5. Its programming model is based on cooperative multitasking and common system programming abstractions such as threads, sockets, files, subprocesses, locks, and queues. However, under the covers it is based on task model provides for advanced handling of cancellation, threads, processes, and much more.

This tutorial will take you through the basics of creating and managing tasks in curio as well as some useful debugging features.

A Small Taste

Curio allows you to write concurrent I/O handling code that looks a lot like threads. For example, here is a simple echo server:

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)
    sock.bind(address)
    sock.listen(5)
    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(1000)
             if not data:
                 break
             await client.sendall(data)
    print('Connection closed')

if __name__ == '__main__':
    run(echo_server(('',25000)))

This server can handle thousands of concurrent clients. It does not use threads. However, this example really doesn’t do Curio justice. In the rest of this tutorial, we’ll start with the basics and work our way back to this.

Getting Started

Here is a simple curio hello world program–a task that prints a simple countdown as you wait for your kid to put their shoes on:

# hello.py
import curio

async def countdown(n):
    while n > 0:
        print('T-minus', n)
        await curio.sleep(1)
        n -= 1

if __name__ == '__main__':
    curio.run(countdown(10))

Run it and you’ll see a countdown. Yes, some jolly fun to be sure. Curio is based around the idea of tasks. Tasks are defined as coroutines using async functions. To make a task execute, it must run inside the curio kernel. The run() function starts the kernel with an initial task. The kernel runs until there are no more tasks to complete.

Tasks

Let’s add a few more tasks into the mix:

# hello.py
import curio

async def countdown(n):
    while n > 0:
        print('T-minus', n)
        await curio.sleep(1)
        n -= 1

async def kid():
    print('Building the Millenium Falcon in Minecraft')
    await curio.sleep(1000)

async def parent():
    kid_task = await curio.spawn(kid())
    await curio.sleep(5)

    print("Let's go")
    count_task = await curio.spawn(countdown(10))
    await count_task.join()

    print("We're leaving!")
    await kid_task.join()
    print('Leaving')

if __name__ == '__main__':
    curio.run(parent())

This program illustrates the process of creating and joining with tasks. Here, the parent() task uses the curio.spawn() coroutine to launch a new child task. After sleeping briefly, it then launches the countdown() task. The join() method is used to wait for a task to finish. In this example, the parent first joins with countdown() and then with kid() before trying to leave. If you run this program, you’ll see it produce the following output:

bash % python3 hello.py
Building the Millenium Falcon in Minecraft
Let's go
T-minus 10
T-minus 9
T-minus 8
T-minus 7
T-minus 6
T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1
We're leaving!
.... hangs ....

At this point, the program appears hung. The child is busy for the next 1000 seconds, the parent is blocked on join() and nothing much seems to be happening–this is the mark of all good concurrent programs (hanging that is). Change the last part of the program to run the kernel with the monitor enabled:

...
if __name__ == '__main__':
    curio.run(parent(), with_monitor=True)

Run the program again. You’d really like to know what’s happening? Yes? Open up another terminal window and connect to the monitor as follows:

bash % python3 -m curio.monitor
Curio Monitor: 3 tasks running
Type help for commands
curio >

See what’s happening by typing ps:

curio > ps
Task   State        Cycles     Timeout Task
------ ------------ ---------- ------- --------------------------------------------------
1      FUTURE_WAIT  2          None    Monitor.monitor_task
2      TASK_JOIN    5          None    parent
3      TIME_SLEEP   1          None    kid
curio >

In the monitor, you can see a list of the active tasks. You can see that the parent is waiting to join and that the kid is sleeping. Actually, you’d like to know more about what’s happening. You can get the stack trace of any task using the where command:

curio > where 2
Stack for Task(id=2, <coroutine object parent at 0x10dda1780>, state='TASK_JOIN') (most recent call last):
  File "hello.py", line 23, in parent
    await kid_task.join()
  File "/Users/beazley/Desktop/Projects/curio/curio/task.py", line 58, in join
    await _join_task(self)
  File "/Users/beazley/Desktop/Projects/curio/curio/traps.py", line 79, in _join_task
    yield ('_trap_join_task', task)

curio > where 3
Stack for Task(id=3, <coroutine object kid at 0x10dda19e8>, state='TIME_SLEEP') (most recent call last):
  File "hello.py", line 12, in kid
    await curio.sleep(1000)
  File "/Users/beazley/Desktop/Projects/curio/curio/task.py", line 95, in sleep
    await _sleep(seconds)
  File "/Users/beazley/Desktop/Projects/curio/curio/traps.py", line 52, in _sleep
    yield ('_trap_sleep', seconds)

curio >

Actually, that kid is just being super annoying. Let’s cancel their world:

curio > cancel 3
Cancelling task 3
*** Connection closed by remote host ***

This causes the whole program to die with a rather nasty traceback message like this:

Curio: Task Crash: parent
Traceback (most recent call last):
  File "/Users/beazley/Desktop/Projects/curio/curio/kernel.py", line 533, in run
    trap = current._throw(current.next_exc)
  File "hello.py", line 12, in kid
    await curio.sleep(1000)
  File "/Users/beazley/Desktop/Projects/curio/curio/task.py", line 95, in sleep
    await _sleep(seconds)
  File "/Users/beazley/Desktop/Projects/curio/curio/traps.py", line 52, in _sleep
    yield ('_trap_sleep', seconds)
curio.errors.CancelledError: CancelledError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/beazley/Desktop/Projects/curio/curio/kernel.py", line 531, in run
    trap = current._send(current.next_value)
  File "hello.py", line 23, in parent
    await kid_task.join()
  File "/Users/beazley/Desktop/Projects/curio/curio/task.py", line 60, in join
   raise TaskError('Task crash') from self.exc_info[1]
curio.errors.TaskError: Task crash
bash %

Not surprisingly, the parent sure didn’t like having their child process abrubtly killed like that. The join() method returned with a TaskError exception to indicate that some kind of problem occurred in the child.

Debugging is an important feature of curio and by using the monitor, you see what’s happening as tasks run. You can find out where tasks are blocked and you can cancel any task that you want. However, it’s not necessary to do this in the monitor. Change the parent task to include a timeout and a cancellation request like this:

async def parent():
    kid_task = await curio.spawn(kid())
    await curio.sleep(5)

    print("Let's go")
    count_task = await curio.spawn(countdown(10))
    await count_task.join()

    print("We're leaving!")
    try:
        await curio.timeout_after(10, kid_task.join())
    except curio.TaskTimeout:
        print('I warned you!')
        await kid_task.cancel()
    print('Leaving!')

If you run this version, the parent will wait 10 seconds for the child to join. If not, the child is forcefully cancelled. Problem solved. Now, if only real life were this easy.

Of course, all is not lost in the child. If desired, they can catch the cancellation request and cleanup. For example:

async def kid():
    try:
        print('Building the Millenium Falcon in Minecraft')
        await curio.sleep(1000)
    except curio.CancelledError:
        print('Fine. Saving my work.')
        raise

Now your program should produce output like this:

bash % python3 hello.py
Building the Millenium Falcon in Minecraft
Let's go
T-minus 10
T-minus 9
T-minus 8
T-minus 7
T-minus 6
T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1
We're leaving!
I warned you!
Fine. Saving my work.
Leaving!

By now, you have the basic gist of the curio task model. You can create tasks, join tasks, and cancel tasks. Even if a task appears to be blocked for a long time, it can be cancelled by another task or a timeout. You have a lot of control over the environment.

Task Synchronization

Although threads are not used to implement curio, you still might have to worry about task synchronization issues (e.g., if more than one task is working with mutable state). For this purpose, curio provides Event, Lock, Semaphore, and Condition objects. For example, let’s introduce an event that makes the child wait for the parent’s permission to start playing:

start_evt = curio.Event()

async def kid():
    print('Can I play?')
    await start_evt.wait()
    try:
        print('Building the Millenium Falcon in Minecraft')
        await curio.sleep(1000)
    except curio.CancelledError:
        print('Fine. Saving my work.')
        raise

async def parent():
    kid_task = await curio.spawn(kid())
    await curio.sleep(5)

    print('Yes, go play')
    await start_evt.set()
    await curio.sleep(5)

    print("Let's go")
    count_task = await curio.spawn(countdown(10))
    await count_task.join()

    print("We're leaving!")
    try:
        await curio.timeout_after(10, kid_task.join())
    except curio.TaskTimeout:
        print('I warned you!')
        await kid_task.cancel()
    print('Leaving!')

All of the synchronization primitives work the same way that they do in the threading module. The main difference is that all operations must be prefaced by await. Thus, to set an event you use await start_evt.set() and to wait for an event you use await start_evt.wait().

All of the synchronization methods also support timeouts. So, if the kid wanted to be rather annoying, they could use a timeout to repeatedly nag like this:

async def kid():
    while True:
        try:
            print('Can I play?')
            await curio.timeout_after(1, start_evt.wait())
            break
        except curio.TaskTimeout:
            print('Wha!?!')
    try:
        print('Building the Millenium Falcon in Minecraft')
        await curio.sleep(1000)
    except curio.CancelledError:
        print('Fine. Saving my work.')
        raise

Signals

What kind of helicopter parent lets their child play Minecraft for a measly 5 seconds? Instead, let’s have the parent allow the child to play as much as they want until a Unix signal arrives, indicating that it’s time to go. Modify the code to wait on a SignalSet like this:

import signal, os

async def parent():
    print('Parent PID', os.getpid())
    kid_task = await curio.spawn(kid())
    await curio.sleep(5)

    print('Yes, go play')
    await start_evt.set()

    await curio.SignalSet(signal.SIGHUP).wait()

    print("Let's go")
    count_task = await curio.spawn(countdown(10))
    await count_task.join()
    print("We're leaving!")
    try:
        await curio.timeout_after(10, kid_task.join())
    except curio.TaskTimeout:
        print('I warned you!')
        await kid_task.cancel()
    print('Leaving!')

If you run this program, the parent lets the kid play indefinitely–well, until a SIGHUP arrives. When you run the program, you’ll see this:

bash % python3 hello.py
Parent PID 36069
Can I play?
Wha!?!
Can I play?
Wha!?!
Can I play?
Wha!?!
Can I play?
Wha!?!
Can I play?
Yes, go play
Building the Millenium Falcon in Minecraft

Don’t forget, if you’re wondering what’s happening, you can always go to a different terminal window and drop into the curio monitor:

bash % python3 -m curio.monitor

Curio Monitor: 3 tasks running
Type help for commands
curio > ps
Task   State        Cycles     Timeout Task
------ ------------ ---------- ------- --------------------------------------------------
1      FUTURE_WAIT  2          None    Monitor.monitor_task
2      SIGNAL_WAIT  5          None    parent
3      TIME_SLEEP   16         None    kid
curio >

Here you see the parent waiting on a signal and the kid sleeping. If you want to initiate the signal, go to a separate terminal and type this:

bash % kill -HUP 36069

Alternatively, you can initiate the signal by typing this in the monitor:

curio > signal SIGHUP

In either case, you’ll see the parent wake up, do the countdown and proceed to cancel the child. Very good.

Number Crunching and Blocking Operations

Now, suppose for a moment that the kid has decided, for reasons unknown, that building the Millenium Falcon requires computing a sum of larger and larger Fibonacci numbers using an exponential algorithm like this:

def fib(n):
    if n <= 2:
        return 1
    else:
        return fib(n-1) + fib(n-2)

async def kid():
    print('Can I play?')
    await start_evt.wait()
    try:
        print('Building the Millenium Falcon in Minecraft')
        total = 0
        for n in range(50):
             total += fib(n)
    except curio.CancelledError:
        print('Fine. Saving my work.')
        raise

If you run this version, you’ll find that the entire kernel becomes unresponsive. For example, signals aren’t caught and there appears to be no way to get control back. The problem here is that the kid is hogging the CPU and never yields. Important lesson: curio does not provide preemptive scheduling. If a task decides to compute large Fibonacci numbers or mine bitcoins, everything will block until it’s done. Don’t do that.

If you know that work might take awhile, you can have it execute in a separate process. Change the code to use curio.run_in_process() like this:

async def kid():
    print('Can I play?')
    await start_evt.wait()
    try:
        print('Building the Millenium Falcon in Minecraft')
        total = 0
        for n in range(50):
            total += await curio.run_in_process(fib, n)
    except curio.CancelledError:
        print('Fine. Saving my work.')
        raise

In this version, the kernel remains fully responsive because the CPU intensive work is being carried out in a subprocess. You should be able to run the monitor, send the signal, and see the shutdown occur as before.

The problem of blocking might also apply to other operations involving I/O. For example, accessing a database or calling out to other libraries. In fact, any I/O operation not preceded by an explicit await might block. If you know that blocking is possible, use the curio.run_in_thread() coroutine. This arranges to have the computation carried out in a separate thread. For example:

import time

async def kid():
    print('Can I play?')
    await start_evt.wait()
    try:
        print('Building the Millenium Falcon in Minecraft')
        total = 0
        for n in range(50):
            total += await curio.run_in_process(fib, n)
            # Rest for a bit
            await curio.run_in_thread(time.sleep, n)
    except curio.CancelledError:
        print('Fine. Saving my work.')

Note: time.sleep() has only been used to illustrate blocking in an outside library. curio already has its own sleep function so if you really need to sleep, use that instead.

A Simple Echo Server

Now that you’ve got the basics down, let’s look at some I/O. Perhaps the main use of Curio is in network programming. Here is a simple echo server written directly with sockets using 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)
    sock.bind(address)
    sock.listen(5)
    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(1000)
             if not data:
                 break
             await client.sendall(data)
    print('Connection closed')

if __name__ == '__main__':
    run(echo_server(('',25000)))

Run this program and try connecting to it using a command such as nc or telnet. You’ll see the program echoing back data to you. Open up multiple connections and see that it handles multiple client connections perfectly well:

bash % nc localhost 25000
Hello                 (you type)
Hello                 (response)
Is anyone there?      (you type)
Is anyone there?      (response)
^C
bash %

If you’ve written a similar program using sockets and threads, you’ll find that this program looks nearly identical except for the use of async and await. Any operation that involves I/O, blocking, or the services of the kernel is prefaced by await.

Carefully notice that we are using the module curio.socket instead of the built-in socket module here. Under the covers, curio.socket is a wrapper around the existing socket module. All of the existing functionality of socket is available, but all of the operations that might block have been replaced by coroutines and must be preceded by an explicit await.

The use of an asynchronous context manager might be something new. For example, you’ll notice the code uses this:

async with sock:
    ...

Normally, a context manager takes care of closing a socket when you’re done using it. The same thing happens here. However, because you’re operating in an environment of cooperative multitasking, you should use the asynchronous variant instead. As a general rule, all I/O related operations in curio will use the async form.

A lot of the above code involving sockets is fairly repetitive. Instead of writing the part that sets up the server, you can simplify the above example using tcp_server() like this:

from curio import run, spawn, tcp_server

async def echo_client(client, addr):
    print('Connection from', addr)
    while True:
        data = await client.recv(1000)
        if not data:
            break
        await client.sendall(data)
    print('Connection closed')

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

The tcp_server() coroutine takes care of a few low-level details such as creating the server socket and binding it to an address. It also takes care of properly closing the client socket so you no longer need the extra async with client statement from before.

A Stream-Based Echo Server

In certain cases, it might be easier to work with a socket connection using a file-like stream interface. Here is an example:

from curio import run, spawn, tcp_server

async def echo_client(client, addr):
    print('Connection from', addr)
    s = client.as_stream()
    while True:
        data = await s.read(1000)
        if not data:
            break
        await s.write(data)
    print('Connection closed')

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

The socket.as_stream() method can be used to wrap the socket in a file-like object for reading and writing. On this object, you would now use standard file methods such as read(), readline(), and write(). One feature of a stream is that you can easily read data line-by-line using an async for statement like this:

from curio import run, spawn, tcp_server

async def echo_client(client, addr):
    print('Connection from', addr)
    s = client.as_stream()
    async for line in s:
        await s.write(line)
    print('Connection closed')

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

This is potentially useful if you’re writing code to read HTTP headers or some similar task.

A Managed Echo Server

Let’s make a slightly more sophisticated echo server that responds to a Unix signal:

import signal
from curio import run, spawn, SignalSet, CancelledError, tcp_server, current_task

clients = set()

async def echo_client(client, addr):
    task = await current_task()
    clients.add(task)
    print('Connection from', addr)
    try:
        while True:
            data = await client.recv(1000)
            if not data:
                break
            await client.sendall(data)
        print('Connection closed')
    except CancelledError:
        await client.sendall(b'Server going down\n')
        raise
    finally:
        clients.remove(task)

async def main(host, port):
    while True:
        async with SignalSet(signal.SIGHUP) as sigset:
            print('Starting the server')
            serv_task = await spawn(tcp_server(host, port, echo_client))
            await sigset.wait()
            print('Server shutting down')
            await serv_task.cancel()

            for task in list(clients):
                await task.cancel()

if __name__ == '__main__':
    run(main('', 25000))

In this code, the main() coroutine launches the server, but then waits for the arrival of a SIGHUP signal. When received, it cancels the server and then all children created by the server. An interesting thing about this cancellation is that each child task adds/removes itself from a set of the active children (the clients set). The echo_client() coroutine has been programmed to catch the resulting cancellation exception and perform a clean shutdown, sending a message back to the client that a shutdown is occurring. Just to be clear, if there were a 1000 connected clients at the time the restart occurs, the server would drop all 1000 clients at once and start fresh with no active connections.

Making Connections

Curio provides some high-level functions for making outgoing connections. For example, here is a task that makes a connection to www.python.org:

import curio

async def main():
    sock = await curio.open_connection('www.python.org', 80)
    async with sock:
        await sock.sendall(b'GET / HTTP/1.0\r\nHost: www.python.org\r\n\r\n')
        chunks = []
        while True:
            chunk = await sock.recv(10000)
            if not chunk:
                break
            chunks.append(chunk)

    response = b''.join(chunks)
    print(response.decode('latin-1'))

if __name__ == '__main__':
    curio.run(main())

If you run this, you should get some output that looks similar to this:

HTTP/1.1 301 Moved Permanently
Server: Varnish
Retry-After: 0
Location: https://www.python.org/
Content-Length: 0
Accept-Ranges: bytes
Date: Fri, 30 Oct 2015 17:33:34 GMT
Via: 1.1 varnish
Connection: close
X-Served-By: cache-dfw1826-DFW
X-Cache: HIT
X-Cache-Hits: 0
Strict-Transport-Security: max-age=63072000; includeSubDomains

Ah, a redirect to HTTPS. Let’s make a connection with SSL applied to it:

import curio

async def main():
    sock = await curio.open_connection('www.python.org', 443,
                                       ssl=True,
                                       server_hostname='www.python.org')
    async with sock:
        await sock.sendall(b'GET / HTTP/1.0\r\nHost: www.python.org\r\n\r\n')
        chunks = []
        while True:
            chunk = await sock.recv(10000)
            if not chunk:
                break
            chunks.append(chunk)

    response = b''.join(chunks)
    print(response.decode('latin-1'))

if __name__ == '__main__':
    curio.run(main())

At this point it’s worth noting that the primary purpose of curio is merely concurrency and I/O. You can create sockets and you can apply things such as SSL to them. However, curio doesn’t implement any application-level protocols such as HTTP. Think of curio as a base-layer for doing that.

An SSL Server

Since we’re on the subject of SSL, here’s an example of a server that speaks SSL:

import curio
from curio import ssl
import time

KEYFILE = 'privkey_rsa'       # Private key
CERTFILE = 'certificate.crt'  # Server certificate

async def handler(client, addr):
    client_f = client.as_stream()

    # Read the HTTP request
    async for line in client_f:
       line = line.strip()
       if not line:
           break
       print(line)

    # Send a response
    await client_f.write(
b'''HTTP/1.0 200 OK\r
Content-type: text/plain\r
\r
If you're seeing this, it probably worked. Yay!
''')
    await client_f.write(time.asctime().encode('ascii'))
    await client.close()

if __name__ == '__main__':
    ssl_context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
    ssl_context.load_cert_chain(certfile=CERTFILE, keyfile=KEYFILE)
    curio.run(curio.tcp_server('', 10000, handler, ssl=ssl_context))

The curio.ssl submodule is a wrapper around the ssl module in the standard library. It has been modified slightly so that functions responsible for wrapping sockets return a socket compatible with curio. Otherwise, you’d use it the same way as the normal ssl module.

To test this out, point a browser at https://localhost:10000 and see if you get a readable response. The browser might yell at you with some warnings about the certificate if it’s self-signed or misconfigured in some way. However, the example shows the basic steps involved in using SSL with curio.

Blocking I/O

Normally, all of the I/O you perform in curio will be non-blocking, using functions that make explicit use of await. However, you may encounter situations where you want to interoperate with existing synchronous code outside of curio. To do this, you can temporarily put sockets and streams into blocking mode and expose the raw socket or file underneath. Use the blocking() context manager method as shown here:

from curio import run, spawn, tcp_server

async def echo_client(client, addr):
    print('Connection from', addr)
    while True:
        data = await client.recv(1000)
        if not data:
            break

        # Temporarily enter blocking mode and use as a normal socket
        with client.blocking() as _client:
            _client.sendall(data)

    print('Connection closed')

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

The blocking() method unwraps the low-level socket, places it in blocking mode, and returns it back to you. In this example the _client variable is the raw socket object as created by Python’s socket module. You could pass it to any function that expects to work with a normal socket. Just be aware that any I/O operations on it could potentially block the curio kernel. If you’re not sure, combine your operation with the run_in_thread() function. For example:

from curio import run, spawn, tcp_server, run_in_thread

async def echo_client(client, addr):
    print('Connection from', addr)
    while True:
        data = await client.recv(1000)
        if not data:
            break

        # Temporarily enter blocking mode
        with client.blocking() as _client:
            await run_in_thread(_client.sendall, data)

    print('Connection closed')

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

Normally, you wouldn’t do this for such a operation like sendall(). However, the combination of the blocking() method and run_in_thread() function could be used to implement a hybrid server design where you use curio to coordinate a very large collection of mostly inactive connections and a thread-pool to carry operations in previously written synchronous code.

Subprocesses

Curio provides a wrapper around the subprocess module for launching subprocesses. For example, suppose you wanted to write a task to watch the output of the ping command in real time:

from curio import subprocess
import curio

async def main():
    p = subprocess.Popen(['ping', 'www.python.org'],
                         stdout=subprocess.PIPE)
    async for line in p.stdout:
        print('Got:', line.decode('ascii'), end='')

if __name__ == '__main__':
    curio.run(main())

In addition to Popen(), you can also use higher level functions such as subprocess.run() and subprocess.check_output(). For example:

from curio import subprocess
async def main():
    try:
        out = await subprocess.check_output(['netstat', '-a'])
    except subprocess.CalledProcessError as e:
        print('It failed!', e)

These functions operate exactly as they do in the normal subprocess module except that they’re written on top of the curio kernel. There is no blocking and no use of hidden threads.

Intertask Communication

If you have multiple tasks and want them to communicate, use a Queue. For example:

# prodcons.py

import curio

async def producer(queue):
    for n in range(10):
        await queue.put(n)
    await queue.join()
    print('Producer done')

async def consumer(queue):
    while True:
        item = await queue.get()
        print('Consumer got', item)
        await queue.task_done()

async def main():
    q = curio.Queue()
    prod_task = await curio.spawn(producer(q))
    cons_task = await curio.spawn(consumer(q))
    await prod_task.join()
    await cons_task.cancel()

if __name__ == '__main__':
    curio.run(main())

Curio provides the same synchronization primitives as found in the built-in threading module. The same techniques used by threads can be used with curio.

Task-local storage

Sometimes it happens that you want to store some data that is specific to a particular Task in a place where it can be reached from anywhere, without having to pass it around everywhere. For example, in a server that responds to network requests, you might want to assign each request a unique tag, and then make sure to include that unique tag in all log messages generated while handling the request. If we were using threads, the solution would be thread-local storage implemented with threading.local. In Curio, we use task-local storage, implemented by curio.Local. For example:

# local-example.py

import curio

import random
r = random.Random(0)

request_info = curio.Local()

# Example logging function that tags each line with the request identifier.
def log(msg):
    # Read from task-local storage:
    request_tag = request_info.tag

    print("request {}: {}".format(request_tag, msg))

async def concurrent_helper(job):
    log("running helper task {}".format(job))
    await curio.sleep(r.random())
    log("finished helper task {}".format(job))

async def handle_request(tag):
    # Write to task-local storage:
    request_info.tag = tag

    log("Request received")
    await curio.sleep(r.random())
    helpers = [
        await curio.spawn(concurrent_helper(1)),
        await curio.spawn(concurrent_helper(2)),
    ]
    for helper in helpers:
        await helper.join()
    await curio.sleep(r.random())
    log("Request complete")

async def main():
    tasks = []
    for i in range(3):
        tasks.append(await curio.spawn(handle_request(i)))
    for task in tasks:
        await task.join()

if __name__ == "__main__":
    curio.run(main())

which produces output like:

request 0: Request received
request 1: Request received
request 2: Request received
request 2: running helper task 1
request 2: running helper task 2
request 2: finished helper task 1
request 1: running helper task 1
request 1: running helper task 2
request 0: running helper task 1
request 0: running helper task 2
request 2: finished helper task 2
request 0: finished helper task 1
request 1: finished helper task 1
request 0: finished helper task 2
request 2: Request complete
request 1: finished helper task 2
request 1: Request complete
request 0: Request complete

Notice two features in particular:

  • Unlike almost all other APIs in curio, accessing task-local storage does not use await. As an example of why this is useful, imagine you wanted to capture logs written via the standard library logging module, and annotate them with request identifiers. Because logging is synchronous, this would be impossible if accessing task-local storage required await.
  • Unlike threading.local, Curio task-local variables are inherited. Notice how in our example above, the logs from concurrent_helper are tagged with the appropriate request.

Programming Advice

At this point, you should have enough of the core concepts to get going. Here are a few programming tips to keep in mind:

  • When writing code, think thread programming and synchronous code. Tasks execute like threads and would need to be synchronized in much the same way. However, unlike threads, tasks can only be preempted on statements that explicitly use await or async.
  • Curio uses the same I/O abstractions that you would use in normal synchronous code (e.g., sockets, files, etc.). Methods have the same names and perform the same functions. However, all operations that potentially involve I/O or blocking will always be prefaced by an explicit await keyword.
  • Be extra wary of any library calls that do not use an explicit await. Although these calls will work, they could potentially block the kernel on I/O or long-running calculations. If you know that either of these are possible, consider the use of the run_in_process() or run_in_thread() functions to execute the work.

Debugging Tips

A common programming mistake is to forget to use await. For example:

async def countdown(n):
    while n > 0:
        print('T-minus', n)
        curio.sleep(5)        # Missing await
        n -= 1

This will usually result in a warning message:

example.py:8: RuntimeWarning: coroutine 'sleep' was never awaited

Another possible source of failure involves attempts to use curio-wrapped sockets and files with existing synchronous code. Doing so might result in a TypeError or some kind of problem related to non-blocking behavior. If you need to interoperate with external code, make sure you use the blocking() method to expose the raw socket or file being used behind the scenes. For example:

# sock is a curio socket
with sock.blocking() as _sock:
    external_function(_sock)       # Pass to external function
    ...

For debugging a program that is otherwise running, but you’re not exactly sure what it might be doing (perhaps it’s hung or deadlocked), consider the use of the curio monitor. For example:

import curio
...
run(..., with_monitor=True)

The monitor can show you the state of each task and you can get stack traces. Remember that you enter the monitor by running python3 -m curio.monitor in a separate window.

More Information

The official Github page at https://github.com/dabeaz/curio should be used for bug reports, pull requests, and other activities.

A reference manual can be found at https://curio.readthedocs.io/en/latest/reference.html.

A more detailed developer’s guide can be found at https://curio.readthedocs.io/en/latest/devel.html.