Curio Reference Manual

This manual describes the basic concepts and functionality provided by curio.

Coroutines

Curio is solely concerned with the execution of coroutines. A coroutine is a function defined using async def. For example:

async def hello(name):
      return 'Hello ' + name

Coroutines call other coroutines using await. For example:

async def main():
      s = await hello('Guido')
      print(s)

Unlike a normal function, a coroutine can never run all on its own. It always has to execute under the supervision of a manager (e.g., an event-loop, a kernel, etc.). In curio, an initial coroutine is executed by a low-level kernel using the run() function. For example:

import curio
curio.run(main())

When executed by curio, a coroutine is considered to be a “Task.” Whenever the word “task” is used, it refers to the execution of a coroutine.

The Kernel

All coroutines in curio are executed by an underlying kernel. Normally, you would run a top-level coroutine using the following function:

run(coro, *, log_errors=True, selector=None,

with_monitor=False,

warn_if_task_blocks_for=None, **other_kernel_args)

Run the coroutine coro to completion and return its final return value. If log_errors is True, a traceback is written to the log on crash. If with_monitor is True, then the monitor debugging task executes in the background. If selector is given, it should be an instance of a selector from the selectors module. If warn_if_task_blocks_for is given, then any tasks which blocks the event loop for the given number of seconds will trigger a curio.BlockingTaskWarning.

If you are going to repeatedly run coroutines one after the other, it will be more efficient to create a Kernel instance and submit them using its run() method as described below:

class Kernel(log_errors=True, selector=None, with_monitor=False)

Create an instance of a curio kernel. The arguments are the same as described above for the run() function.

There is only one method that may be used on a Kernel outside of coroutines.

Kernel.run(coro=None, *, shutdown=False)

Runs the kernel until all non-daemonic tasks have finished execution. coro is a coroutine to run as a task. If shutdown is True, the kernel will cancel all daemonic tasks and perform a clean shutdown once all regular tasks have completed. Calling this method with no coroutine and shutdown set to True will make the kernel cancel all remaining tasks and perform a clean shut down.

If submitting multiple tasks, one after another, from synchronous code, consider using a kernel as a context manager. For example:

with Kernel() as kernel:
    kernel.run(coro1())
    kernel.run(coro2())
    ...
# Kernel shuts down here

Tasks

The following functions are defined to help manage the execution of tasks.

await spawn(coro, daemon=False)

Create a new task that runs the coroutine coro. Returns a Task instance as a result. The daemon option, if supplied, specifies that the new task will run indefinitely in the background. Curio only runs as long as there are non-daemonic tasks to execute. Note: a daemonic task will still be cancelled if the underlying kernel is shut down.

await current_task()

Returns a reference to the Task instance corresponding to the caller. A coroutine can use this to get a self-reference to its current Task instance if needed.

The spawn() and current_task() both return a Task instance that serves as a kind of wrapper around the underlying coroutine that’s executing.

class Task

A class representing an executing coroutine. This class cannot be created directly.

await Task.join()

Wait for the task to terminate. Returns the value returned by the task or raises a curio.TaskError exception if the task failed with an exception. This is a chained exception. The __cause__ attribute of this exception contains the actual exception raised by the task when it crashed. If called on a task that has been cancelled, the __cause__ attribute is set to curio.TaskCancelled.

await Task.cancel(blocking=True)

Cancels the task. This raises a curio.TaskCancelled exception in the task which may choose to handle it in order to perform cleanup actions. If blocking=True (the default), does not return until the task actually terminates. Curio only allows a task to be cancelled once. If this method is somehow invoked more than once on a still running task, the second request will merely wait until the task is cancelled from the first request. If the task has already run to completion, this method does nothing and returns immediately. Returns True if the task was actually cancelled. False is returned if the task was already finished prior to the cancellation request. Cancelling a task also cancels any previously set timeout.

The following public attributes are available of Task instances:

Task.id

The task’s integer id.

Task.coro

The underlying coroutine associated with the task.

Task.daemon

Boolean flag that indicates whether or not a task is daemonic.

Task.state

The name of the task’s current state. Printing it can be potentially useful for debugging.

Task.cycles

The number of scheduling cycles the task has completed. This might be useful if you’re trying to figure out if a task is running or not. Or if you’re trying to monitor a task’s progress.

Task.exc_info

A tuple of exception information obtained from sys.exc_info() if the task crashes for some reason. Potentially useful for debugging.

Task.cancelled

A boolean flag that indicates whether or not the task was cancelled.

Task.terminated

A boolean flag that indicates whether or not the task has run to completion.

Task local storage

Curio supports “task local storage”. The API is modeled after the “thread local storage” provided by threading.local.

class Local

A class representing a bundle of task-local values. Objects of this class have no particular attributes or methods. Instead, they serve as a blank slate to which you can add whatever attributes you like. Modifications made from within one task will only be visible to that task – with one exception: when you create a new task using curio.spawn, then any values assigned to Local objects in the parent task will be inherited by the child. This inheritance takes the form of a shallow copy – further changes in the parent won’t affect the child, and further changes in the child won’t affect the parent.

Time

The following functions are used by tasks to help manage time.

await sleep(seconds)

Sleep for a specified number of seconds. If the number of seconds is 0, the kernel merely switches to the next task (if any).

await wake_at(clock)

Sleep until the monotonic clock reaches the given absolute clock value. Returns the value of the monotonic clock at the time the task awakes. Use this function if you need to have more precise interval timing.

await clock()

Returns the current value of the kernel clock. This is often used in conjunction with the wake_at() function (you’d use this to get an initial clock value for passing an argument).

Timeouts

Any blocking operation in curio can be cancelled after a timeout. The following functions can be used for this purpose:

await timeout_after(seconds, coro=None)

Execute the specified coroutine and return its result. However, issue a cancellation request to the calling task after seconds have elapsed. When this happens, a curio.TaskTimeout exception is raised. If coro is None, the result of this function serves as an asynchronous context manager that applies a timeout to a block of statements.

timeout_after() may be composed with other timeout_after() operations (i.e., nested timeouts). If an outer timeout expires first, then curio.TimeoutCancellationError is raised instead of curio.TaskTimeout. If an inner timeout expires and fails to properly catch curio.TaskTimeout, a curio.UncaughtTimeoutError is raised in the outer timeout.

await ignore_after(seconds, coro=None, *, timeout_result=None)

Execute the specified coroutine and return its result. Issue a cancellation request after seconds have elapsed. When a timeout occurs, no exception is raised. Instead, None or the value of timeout_result is returned. If coro is None, the result is an asynchronous context manager that applies a timeout to a block of statements. For the context manager case, result attribute of the manager is set to None or the value of timeout_result if the block was cancelled.

Note: ignore_after() may also be composed with other timeout operations. curio.TimeoutCancellationError and curio.UncaughtTimeoutError exceptions might be raised according to the same rules as for timeout_after().

Here is an example that shows how these functions can be used:

# Execute coro(args) with a 5 second timeout
try:
    result = await timeout_after(5, coro(args))
except TaskTimeout as e:
    result = None

# Execute multiple statements with a 5 second timeout
try:
    async with timeout_after(5):
         await coro1(args)
         await coro2(args)
         ...
except TaskTimeout as e:
    # Handle the timeout
    ...

The difference between timeout_after() and ignore_after() concerns the exception handling behavior when time expires. The latter function returns None instead of raising an exception which might be more convenient in certain cases. For example:

result = await ignore_after(5, coro(args))
if result is None:
    # Timeout occurred (if you care)
    ...

# Execute multiple statements with a 5 second timeout
async with ignore_after(5) as s:
    await coro1(args)
    await coro2(args)
    ...
    s.result = successful_result

if s.result is None:
    # Timeout occurred

It’s important to note that every curio operation can be cancelled by timeout. Rather than having every possible call take an explicit timeout argument, you should wrap the call using timeout_after() or ignore_after() as appropriate.

Cancellation Control

disable_cancellation(coro=None)

Disables the delivery of cancellation-related exceptions to the calling task. Cancellations will be delivered to the first blocking operation that’s performed once cancellation delivery is reenabled. This function may be used to shield a single coroutine or used as a context manager (see example below).

enable_cancellation(coro=None)

Reenables the delivery of cancellation-related exceptions. This function is used as a context manager. It may only be used inside a context in which cancellation has been disabled. This function may be used to shield a single coroutine or used as a context manager (see example below).

await check_cancellation()

Checks to see if any cancellation is pending for the calling task. If cancellation is allowed, a cancellation exception is raised immediately. If cancellation is not allowed, it returns the pending cancellation exception instance (if any). Returns None if no cancellation is pending.

Use of these functions is highly specialized and is probably best avoided. Here is an example that shows typical usage:

async def coro():
    async with disable_cancellation():
        while True:
            await coro1()
            await coro2()
            async with enable_cancellation():
                await coro3()   # May be cancelled
                await coro4()   # May be cancelled

            if await check_cancellation():
                break   # Bail out!

    await blocking_op()   # Cancellation (if any) delivered here

If you only need to shield a single operation, you can write statements like this:

async def coro():
    ...
    await disabled_cancellation(some_operation())
    ...

This is shorthand for writing the following:

async def coro():
    ...
    async with disable_cancellation():
        await some_operation()
    ...

See the section on cancellation in the Curio Developer’s Guide for more detailed information.

Performing External Work

Sometimes you need to perform work outside the kernel. This includes CPU-intensive calculations and blocking operations. Use the following functions to do that:

await curio.workers.run_in_process(callable, *args, **kwargs)

Run callable(*args, **kwargs) in a separate process and returns the result. If cancelled, the underlying worker process (if started) is immediately cancelled by a SIGTERM signal.

await curio.workers.run_in_thread(callable, *args, **kwargs)

Run callable(*args, **kwargs) in a separate thread and return the result. If the calling task is cancelled, the underlying worker thread (if started) is set aside and sent a termination request. However, since there is no underlying mechanism to forcefully kill threads, the thread won’t recognize the termination request until it runs the requested work to completion. It’s important to note that a cancellation won’t block other tasks from using threads. Instead, cancellation produces a kind of “zombie thread” that executes the requested work, discards the result, and then disappears. For reliability, work submitted to threads should have a timeout or some other mechanism that puts a bound on execution time.

await curio.workers.block_in_thread(callable, *args, **kwargs)

The same as run_in_thread(), but guarantees that only one background thread is used for each unique callable regardless of how many tasks simultaneously try to carry out the same operation at once. Only use this function if there is an expectation that the provided callable is going to block for an undetermined amount of time and that there might be a large amount of contention from multiple tasks on the same resource. The primary use is on waiting operations involving foreign locks and queues. For example, if you launched a hundred Curio tasks and they all decided to block on a shared thread queue, using this would be much more efficient than run_in_thread().

await curio.workers.run_in_executor(exc, callable, *args, **kwargs)

Run callable(*args, **kwargs) callable in a user-supplied executor and returns the result. exc is an executor from the concurrent.futures module in the standard library. This executor is expected to implement a submit() method that executes the given callable and returns a Future instance for collecting its result.

When performing external work, it’s almost always better to use the run_in_process() and run_in_thread() functions instead of run_in_executor(). These functions have no external library dependencies, have substantially less communication overhead, and more predictable cancellation semantics.

The following values in curio.workers define how many worker threads and processes are used. If you are going to change these values, do it before any tasks are executed.

curio.workers.MAX_WORKER_THREADS

Specifies the maximum number of threads used by a single kernel using the run_in_thread() function. Default value is 64.

curio.workers.MAX_WORKER_PROCESSES

Specifies the maximum number of processes used by a single kernel using the run_in_process() function. Default value is the number of CPUs on the host system.

I/O Layer

I/O in curio is performed by classes in curio.io that wrap around existing sockets and streams. These classes manage the blocking behavior and delegate their methods to an existing socket or file.

Socket

The Socket class is used to wrap existing an socket. It is compatible with sockets from the built-in socket module as well as SSL-wrapped sockets created by functions by the built-in ssl module. Sockets in curio should be fully compatible most common socket features.

class curio.io.Socket(sockobj)

Creates a wrapper the around an existing socket sockobj. This socket is set in non-blocking mode when wrapped. sockobj is not closed unless the created instance is explicitly closed or used as a context manager.

The following methods are redefined on Socket objects to be compatible with coroutines. Any socket method not listed here will be delegated directly to the underlying socket. Be aware that not all methods have been wrapped and that using a method not listed here might block the kernel or raise a BlockingIOError exception.

await Socket.recv(maxbytes, flags=0)

Receive up to maxbytes of data.

await Socket.recv_into(buffer, nbytes=0, flags=0)

Receive up to nbytes of data into a buffer object.

await Socket.recvfrom(maxsize, flags=0)

Receive up to maxbytes of data. Returns a tuple (data, client_address).

await Socket.recvfrom_into(buffer, nbytes=0, flags=0)

Receive up to nbytes of data into a buffer object.

await Socket.recvmsg(bufsize, ancbufsize=0, flags=0)

Receive normal and ancillary data.

await Socket.recvmsg_into(buffers, ancbufsize=0, flags=0)

Receive normal and ancillary data.

await Socket.send(data, flags=0)

Send data. Returns the number of bytes of data actually sent (which may be less than provided in data).

await Socket.sendall(data, flags=0)

Send all of the data in data. If cancelled, the bytes_sent attribute of the resulting exception contains the actual number of bytes sent.

await Socket.sendto(data, address)

await Socket.sendto(data, flags, address)

Send data to the specified address.

await Socket.sendmsg(buffers, ancdata=(), flags=0, address=None)

Send normal and ancillary data to the socket.

await Socket.accept()

Wait for a new connection. Returns a tuple (sock, address).

await Socket.connect(address)

Make a connection.

await Socket.connect_ex(address)

Make a connection and return an error code instead of raising an exception.

await Socket.writeable()

Waits until the socket is writeable.

await Socket.close()

Close the connection. This method is not called on garbage collection.

await curio.io.do_handshake()

Perform an SSL client handshake. The underlying socket must have already be wrapped by SSL using the curio.ssl module.

Socket.makefile(mode, buffering=0)

Make a file-like object that wraps the socket. The resulting file object is a curio.io.FileStream instance that supports non-blocking I/O. mode specifies the file mode which must be one of 'rb' or 'wb'. buffering specifies the buffering behavior. By default unbuffered I/O is used. Note: It is not currently possible to create a stream with Unicode text encoding/decoding applied to it so those options are not available. If you are trying to put a file-like interface on a socket, it is usually better to use the Socket.as_stream() method below.

Socket.as_stream()

Wrap the socket as a stream using curio.io.SocketStream. The result is a file-like object that can be used for both reading and writing on the socket.

Socket.blocking()

A context manager that temporarily places the socket into blocking mode and returns the raw socket object used internally. This can be used if you need to pass the socket to existing synchronous code.

Socket objects may be used as an asynchronous context manager which cause the underlying socket to be closed when done. For example:

async with sock:
    # Use the socket
    ...
# socket closed here

FileStream

The FileStream class puts a non-blocking wrapper around an existing file-like object. Certain other functions in curio use this (e.g., the Socket.makefile() method).

class curio.io.FileStream(fileobj)

Create a file-like wrapper around an existing file. fileobj must be in in binary mode. The file is placed into non-blocking mode using os.set_blocking(fileobj.fileno()). fileobj is not closed unless the resulting instance is explicitly closed or used as a context manager.

The following methods are available on instances of FileStream:

await FileStream.read(maxbytes=- 1)

Read up to maxbytes of data on the file. If omitted, reads as much data as is currently available and returns it.

await FileStream.readall()

Return all of the data that’s available on a file up until an EOF is read.

await FileStream.readline()

Read a single line of data from a file.

await FileStream.readlines()

Read all of the lines from a file. If cancelled, the lines_read attribute of the resulting exception contains all of the lines that were read so far.

await FileStream.write(bytes)

Write all of the data in bytes to the file.

await FileStream.writelines(lines)

Writes all of the lines in lines to the file. If cancelled, the bytes_written attribute of the exception contains the total bytes written so far.

await FileStream.flush()

Flush any unwritten data from buffers to the file.

await FileStream.close()

Flush any unwritten data and close the file. This method is not called on garbage collection.

FileStream.blocking()

A context manager that temporarily places the stream into blocking mode and returns the raw file object used internally. This can be used if you need to pass the file to existing synchronous code.

Other file methods (e.g., tell(), seek(), etc.) are available if the supplied fileobj also has them.

A FileStream may be used as an asynchronous context manager. For example:

async with stream:
    #  Use the stream object
    ...
# stream closed here

SocketStream

The SocketStream class puts a non-blocking file-like interface around a socket. This is normally created by the Socket.as_stream() method.

class curio.io.SocketStream(sock)

Create a file-like wrapper around an existing socket. sock must be a socket instance from Python’s built-in socket module. The socket is placed into non-blocking mode. sock is not closed unless the resulting instance is explicitly closed or used as a context manager.

A SocketStream instance supports the same methods as FileStream above. One subtle issue concerns the blocking() method below.

SocketStream.blocking()

A context manager that temporarily places the stream into blocking mode and returns a raw file object that wraps the underlying socket. It is important to note that the return value of this operation is a file created open(sock.fileno(), 'rb+', closefd=False). You can pass this object to code that is expecting to work with a file. The file is not closed when garbage collected.

socket wrapper module

The curio.socket module provides a wrapper around the built-in socket module–allowing it to be used as a stand-in in curio-related code. The module provides exactly the same functionality except that certain operations have been replaced by coroutine equivalents.

curio.socket.socket(family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None)

Creates a curio.io.Socket wrapper the around socket objects created in the built-in socket module. The arguments for construction are identical and have the same meaning. The resulting socket instance is set in non-blocking mode.

The following module-level functions have been modified so that the returned socket objects are compatible with curio:

curio.socket.socketpair(family=AF_UNIX, type=SOCK_STREAM, proto=0)

curio.socket.fromfd(fd, family, type, proto=0)

curio.socket.create_connection(address, source_address)

The following module-level functions have been redefined as coroutines so that they don’t block the kernel when interacting with DNS:

await curio.socket.getaddrinfo(host, port, family=0, type=0, proto=0, flags=0)

await curio.socket.getfqdn(name)

await curio.socket.gethostbyname(hostname)

await curio.socket.gethostbyname_ex(hostname)

await curio.socket.gethostname()

await curio.socket.gethostbyaddr(ip_address)

await curio.socket.getnameinfo(sockaddr, flags)

ssl wrapper module

The curio.ssl module provides curio-compatible functions for creating an SSL layer around curio sockets. The following functions are redefined (and have the same calling signature as their counterparts in the standard ssl module:

curio.ssl.wrap_socket(*args, **kwargs)

await curio.ssl.get_server_certificate(*args, **kwargs)

curio.ssl.create_default_context(*args, **kwargs)

class curio.ssl.SSLContext

A redefined and modified variant of ssl.SSLContext so that the wrap_socket() method returns a socket compatible with curio.

Don’t attempt to use the curio.ssl module without a careful read of Python’s official documentation at https://docs.python.org/3/library/ssl.html.

For the purposes of curio, it is usually easier to apply SSL to a connection using some of the high level network functions described in the next section. For example, here’s how you make an outgoing SSL connection:

sock = await curio.open_connection('www.python.org', 443,
                                   ssl=True,
                                   server_hostname='www.python.org')

Here’s how you might define a server that uses SSL:

import curio
from curio import ssl

KEYFILE = "privkey_rsa"       # Private key
CERTFILE = "certificate.crt"  # Server certificat

async def handler(client, addr):
    ...

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

High Level Networking

The following functions are provided to simplify common tasks related to making network connections and writing servers.

await curio.open_connection(host, port, *, ssl=None, source_addr=None, server_hostname=None, alpn_protocols=None)

Creates an outgoing connection to a server at host and port. This connection is made using the socket.create_connection() function and might be IPv4 or IPv6 depending on the network configuration (although you’re not supposed to worry about it). ssl specifies whether or not SSL should be used. ssl can be True or an instance of curio.ssl.SSLContext. source_addr specifies the source address to use on the socket. server_hostname specifies the hostname to check against when making SSL connections. It is highly advised that this be supplied to avoid man-in-the-middle attacks. alpn_protocols specifies a list of protocol names for use with the TLS ALPN extension (RFC7301). A typical value might be ['h2', 'http/1.1'] for negotiating either a HTTP/2 or HTTP/1.1 connection.

await curio.open_unix_connection(path, *, ssl=None, server_hostname=None, alpn_protocols=None)

Creates a connection to a Unix domain socket with optional SSL applied.

await curio.tcp_server(host, port, client_connected_task, *, family=AF_INET, backlog=100, ssl=None, reuse_address=True, reuse_port=False)

Creates a server for receiving TCP connections on a given host and port. client_connected_task is a coroutine that is to be called to handle each connection. Family specifies the address family and is either socket.AF_INET or socket.AF_INET6. backlog is the argument to the socket.socket.listen() method. ssl specifies an curio.ssl.SSLContext instance to use. reuse_address specifies whether to reuse a previously used port. reuse_port specifies whether to use the SO_REUSEPORT socket option prior to binding.

await curio.unix_server(path, client_connected_task, *, backlog=100, ssl=None)

Creates a Unix domain server on a given path. client_connected_task is a coroutine to execute on each connection. backlog is the argument given to the socket.socket.listen() method. ssl is an optional curio.ssl.SSLContext to use if setting up an SSL connection.

Message Passing and Channels

Curio provides a Channel class that can be used to perform message passing between interpreters running in separate processes.

class curio.channel.Channel(address, family=socket.AF_INET)

Represents a communications endpoint for message passing. address is the address and family is the protocol family.

The following methods are used to establish a connection on a Channel instance.

await Channel.accept(*, authkey=None)

Wait for an incoming connection. authkey is an optional authentication key that can be used to authenticate the client. Authentication involves computing an HMAC-based cryptographic digest. The key itself is not transmitted. Returns an Connection instance.

await Channel.connect(*, authkey=None)

Make an outgoing connection. authkey is an optional authentication key. This method repeatedly attempts to make a connection if the other endpoint is not responding. Returns a Connection instance.

Channel.bind()

Performs the address binding step of the accept() method and returns. Can use this if you want the host operating system to assign a port number for you. For example, you can supply an initial address of ('localhost', socket.INADDR_ANY) and call bind(). Afterwards, the address attribute of the Channel instance contains the assigned address.

await Channel.close()

Close the channel.

The connect() and accept() methods of Channel instances return a Connection instance.

class curio.channel.Connection(reader, writer)

Represents a connection on which message passing of Python objects is supported. reader and writer are Curio I/O streams on which reading and writing are to take place.

Instances of Connection support the following methods:

await curio.channel.close()

Close the connection by closing both the reader and writer streams.

await curio.channel.recv()

Receive a Python object. The received object is unserialized using the pickle module.

await curio.channel.recv_bytes(maxlength=None)

Receive a raw message of bytes. maxlength specifies a maximum message size. By default messages may be of arbitrary size.

await curio.channel.send(obj)

Send a Python object. The object must be compatible with the pickle module.

await curio.channel.send_bytes(buf, offset=0, size=None)

Send a buffer of bytes as a single message. offset and size specify an optional byte offset and size into the underlying memory buffer.

await curio.channel.authenticate_server(authkey)

Authenticate the connection for a server.

await curio.channel.authenticate_client(authkey)

Authenticate the connection for a client.

A Connection instance may also be used as a context manager.

Here is an example of a producer program using channels:

# producer.py
from curio import Channel, run

async def producer(ch):
    c = await ch.accept(authkey=b'peekaboo')
    for i in range(10):
        await c.send(i)
    await c.send(None)   # Sentinel

if __name__ == '__main__':
    ch = Channel(('localhost', 30000))
    run(producer(ch))

Here is an example of a correspoinding consumer program using a channel:

# consumer.py
from curio import Channel, run

async def consumer(ch):
    c = await ch.connect(authkey=b'peekaboo')
    while True:
        msg = await c.recv()
        if msg is None:
            break
        print('Got:', msg)

if __name__ == '__main__':
    ch = Channel(('localhost', 30000))
    run(consumer(ch))

subprocess wrapper module

The curio.subprocess module provides a wrapper around the built-in subprocess module.

class curio.subprocess.Popen(*args, **kwargs)

A wrapper around the subprocess.Popen class. The same arguments are accepted. On the resulting Popen instance, the stdin, stdout, and stderr file attributes have been wrapped by the curio.io.FileStream class. You can use these in an asynchronous context.

Here is an example of using Popen to read streaming output off of a subprocess with curio:

import curio
from curio import subprocess

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__':
    kernel = curio.Kernel()
    kernel.add_task(main())
    kernel.run()

The following methods of Popen have been replaced by asynchronous equivalents:

await Popen.wait()

Wait for a subprocess to exit. Cancellation does not terminate the process.

await Popen.communicate(input=b'')

Communicate with the subprocess, sending the specified input on standard input. Returns a tuple (stdout, stderr) with the resulting output of standard output and standard error. If cancelled, the resulting exception has stdout and stderr attributes that contain the output read prior to cancellation. Cancellation does not terminate the underlying subprocess.

The following functions are also available. They accept the same arguments as their equivalents in the subprocess module:

await curio.subprocess.run(args, stdin=None, input=None, stdout=None, stderr=None, shell=False, check=False)

Run a command in a subprocess. Returns a subprocess.CompletedProcess instance. If cancelled, the underlying process is terminated using the process kill() method. The resulting exception will have stdout and stderr attributes containing output read prior to cancellation.

await curio.subprocess.check_output(args, stdout=None, stderr=None, shell=False)

Run a command in a subprocess and return the resulting output. Raises a subprocess.CalledProcessError exception if an error occurred. The behavior on cancellation is the same as for run().

file wrapper module

One problem concerning coroutines and async concerns access to files on the normal file system. Yes, you can use the built-in open() function, but what happens afterwards is hard to predict. Under the covers, the operating system might have to access a disk drive or perform networking of its own. Either way, the operation might take a long time to complete and while it does, the whole Curio kernel will be blocked. You really don’t want that–especially if the system is under heavy load.

The curio.file module provides an asynchronous compatible replacement for the built-in open() function and associated file objects, should you want to read and write traditional files on the filesystem. The underlying implementation avoids blocking. How this is accomplished is an implementation detail (although threads are used in the initial version).

curio.file.aopen(*args, **kwargs)

Creates a curio.file.AsyncFile wrapper around a traditional file object as returned by Python’s builtin open() function. The arguments are exactly the same as for open(). The returned file object must be used as an asynchronous context manager.

class curio.file.AsyncFile(fileobj)

This class represents an asynchronous file as returned by the aopen() function. Normally, instances are created by the aopen() function. However, it can be wrapped around an already-existing file object that was opened using the built-in open() function.

The following methods are redefined on AsyncFile objects to be compatible with coroutines. Any method not listed here will be delegated directly to the underlying file. These methods take the same arguments as the underlying file object. Be aware that not all of these methods are available on all kinds of files (e.g., read1(), readinto() and similar methods are only available in binary-mode files).

await AsyncFile.read(*args, **kwargs)

await AsyncFile.read1(*args, **kwargs)

await AsyncFile.readline(*args, **kwargs)

await AsyncFile.readlines(*args, **kwargs)

await AsyncFile.readinto(*args, **kwargs)

await AsyncFile.readinto1(*args, **kwargs)

await AsyncFile.write(*args, **kwargs)

await AsyncFile.writelines(*args, **kwargs)

await AsyncFile.truncate(*args, **kwargs)

await AsyncFile.seek(*args, **kwargs)

await AsyncFile.tell(*args, **kwargs)

await AsyncFile.flush()

await AsyncFile.close()

AsyncFile objects should always be used as an asynchronous context manager. For example:

async with aopen(filename) as f:
    # Use the file
    data = await f.read()

AsyncFile objects may also be used with asynchronous iteration. For example:

async with open(filename) as f:
    async for line in f:
        ...

AsyncFile objects are intentionally incompatible with code that uses files in a synchronous manner. Partly, this is to help avoid unintentional errors in your program where blocking might occur with you realizing it. If you know what you’re doing and you need to access the underlying file in synchronous code, use the blocking() context manager like this:

async with open(filename) as f:
    ...
    # Pass to synchronous code (danger: might block)
    with f.blocking() as sync_f:
         # Use synchronous I/O operations
         data = sync_f.read()
         ...

Synchronization Primitives

The following synchronization primitives are available. Their behavior is similar to their equivalents in the threading module. None of these primitives are safe to use with threads created by the built-in threading module.

class Event

An event object.

Event instances support the following methods:

Event.is_set()

Return True if the event is set.

Event.clear()

Clear the event.

await Event.wait()

Wait for the event.

await Event.set()

Set the event. Wake all waiting tasks (if any).

Here is an Event example:

import curio

async def waiter(evt):
    print('Waiting')
    await evt.wait()
    print('Running')

async def main():
    evt = curio.Event()
    # Create a few waiters
    await curio.spawn(waiter(evt))
    await curio.spawn(waiter(evt))
    await curio.spawn(waiter(evt))

    await curio.sleep(5)

    # Set the event. All waiters should wake up
    await evt.set()

curio.run(main)

class Lock

This class provides a mutex lock. It can only be used in tasks. It is not thread safe.

Lock instances support the following methods:

await Lock.acquire()

Acquire the lock.

await Lock.release()

Release the lock.

Lock.locked()

Return True if the lock is currently held.

The preferred way to use a Lock is as an asynchronous context manager. For example:

import curio

async def child(lck):
    async with lck:
        print('Child has the lock')

async def main():
    lck = curio.Lock()
    async with lck:
        print('Parent has the lock')
        await curio.spawn(child(lck))
        await curio.sleep(5)

curio.run(main())

class RLock

This class provides a recursive lock funtionality, that could be acquired multiple times within the same task. The behavior of this lock is identical to the threading.RLock, except that the owner of the lock will be a task, wich acquired it, instead of a thread.

RLock instances support the following methods:

await Lock.acquire()

Acquire the lock, incrementing the recursion by 1. Can be used multiple times within the same task, that owns this lock.

await Lock.release()

Release the lock, decrementing the recursion level by 1. If recursion level reaches 0, the lock is unlocked. Raises RuntimeError if called not by the owner or if lock is not locked.

Lock.locked()

Return True if the lock is currently held, i.e. recursion level is greater than 0.

class Semaphore(value=1)

Create a semaphore. Semaphores are based on a counter. If the count is greater than 0, it is decremented and the semaphore is acquired. Otherwise, the task has to wait until the count is incremented by another task.

class BoundedSemaphore(value=1)

This class is the same as Semaphore except that the semaphore value is not allowed to exceed the initial value.

Semaphores support the following methods:

await Semaphore.acquire()

Acquire the semaphore, decrementing its count. Blocks if the count is 0.

await Semaphore.release()

Release the semaphore, incrementing its count. Never blocks.

Semaphore.locked()

Return True if the Semaphore is locked.

Like locks, semaphores support the async-with statement. A common use of semaphores is to limit the number of tasks performing an operation. For example:

import curio

async def worker(sema):
    async with sema:
        print('Working')
        await curio.sleep(5)

async def main():
     sema = curio.Semaphore(2)     # Allow two tasks at a time

     # Launch a bunch of tasks
     for n in range(10):
         await curio.spawn(worker(sema))

     # After this point, you should see two tasks at a time run. Every 5 seconds.

curio.run(main())

class Condition(lock=None)

Condition variable. lock is the underlying lock to use. If none is provided, then a Lock object is used.

Condition objects support the following methods:

Condition.locked()

Return True if the condition variable is locked.

await Condition.acquire()

Acquire the condition variable lock.

await Condition.release()

Release the condition variable lock.

await Condition.wait()

Wait on the condition variable. This releases the underlying lock.

await Condition.wait_for(predicate)

Wait on the condition variable until a supplied predicate function returns True. predicate is a callable that takes no arguments.

await notify(n=1)

Notify one or more tasks, causing them to wake from the Condition.wait() method.

await notify_all()

Notify all tasks waiting on the condition.

Condition variables are often used to signal between tasks. For example, here is a simple producer-consumer scenario:

import curio
from collections import deque

items = deque()
async def consumer(cond):
    while True:
        async with cond:
            while not items:
                await cond.wait()    # Wait for items
            item = items.popleft()
        print('Got', item)

 async def producer(cond):
     for n in range(10):
          async with cond:
              items.append(n)
              await cond.notify()
          await curio.sleep(1)

 async def main():
     cond = curio.Condition()
     await curio.spawn(producer(cond))
     await curio.spawn(consumer(cond))

 curio.run(main())

Queues

If you want to communicate between tasks, it’s usually much easier to use a Queue instead.

class Queue(maxsize=0)

Creates a queue with a maximum number of elements in maxsize. If not specified, the queue can hold an unlimited number of items.

A Queue instance supports the following methods:

Queue.empty()

Returns True if the queue is empty.

Queue.full()

Returns True if the queue is full.

Queue.qsize()

Return the number of items currently in the queue.

await Queue.get()

Returns an item from the queue.

await Queue.put(item)

Puts an item on the queue.

await Queue.join()

Wait for all of the elements put onto a queue to be processed. Consumers must call Queue.task_done() to indicate completion.

await Queue.task_done()

Indicate that processing has finished for an item. If all items have been processed and there are tasks waiting on Queue.join() they will be awakened.

Here is an example of using queues in a producer-consumer problem:

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()

curio.run(main())

class PriorityQueue(maxsize=0)

Creates a priority queue with a maximum number of elements in maxsize.

In a PriorityQueue items are retrieved in priority order with the lowest priority first:

import curio

async def main():
    q = curio.PriorityQueue()
    await q.put((0, 'highest priority'))
    await q.put((100, 'very low priority'))
    await q.put((3, 'higher priority'))

    while not q.empty():
        print(await q.get())

curio.run(main())

This will output

(0, 'highest priority')
(3, 'higher priority')
(100, 'very low priority')

class LifoQueue(maxsize=0)

A queue with “Last In First Out” retrieving policy

import curio

async def main():
    q = curio.LifoQueue()
    await q.put('first')
    await q.put('second')
    await q.put('last')

    while not q.empty():
        print(await q.get())

curio.run(main())

This will output

last
second
first

Here is an example a producer-consumer problem with a UniversalQueue:

from curio import run, UniversalQueue, spawn, run_in_thread

import time
import threading

# An async task
async def consumer(q):
    print('Consumer starting')
    while True:
        item = await q.get()
        if item is None:
            break
        print('Got:', item)
        await q.task_done()
    print('Consumer done')

# A threaded producer
def producer(q):
    for i in range(10):
        q.put(i)
        time.sleep(1)
    q.join()
    print('Producer done')

async def main():
    q = UniversalQueue()
    t1 = await spawn(consumer(q))
    t2 = threading.Thread(target=producer, args=(q,))
    t2.start()
    await run_in_thread(t2.join)
    await q.put(None)
    await t1.join()
    await q.shutdown()

run(main())

In this code, the consumer() is a Curio task and producer() is a thread.

If the withfd=True option is given to a UniveralQueue, it additionally has a fileno() method that can be passed to various functions that might poll for I/O events. When enabled, putting something in the queue will also write a byte of I/O. This might be useful if trying to pass data from Curio to a foreign event loop.

Synchronizing with Threads and Processes

Curio’s synchronization primitives aren’t safe to use with externel threads or processes. However, Curio can work with existing thread or process-level synchronization primitives if you use the abide() function.

await abide(op, *args, **kwargs)

Execute an operation in a manner that safely works with async code. If op is a coroutine function, then op(*args, **kwargs) is returned. If op is a synchronous function, then block_in_thread(op, *args, **kwargs) is returned. In both cases, you would use await to obtain the result. If op is an asynchronous context manager, it is returned unmodified. If op is a synchronous context manager, it is wrapped in a manner that carries out its execution in a backing thread. For this latter case, a special keyword argument reserve_thread=True may be given that instructs Curio to use the same backing thread for the entire duraction of the context manager.

The main use of this function is in code that wants to safely synchronize curio with threads and processes. For example, here is how you would synchronize a thread with a curio task using a threading lock:

import curio
import threading
import time

# A curio task
async def child(lock):
    async with curio.abide(lock):
        print('Child has the lock')

# A thread
def parent(lock):
    with lock:
         print('Parent has the lock')
         time.sleep(5)

lock = threading.Lock()
threading.Thread(target=parent, args=(lock,)).start()
curio.run(child(lock))

If you wanted to trigger or wait for a thread-event, you might do this:

import curio
import threading

evt = threading.Event()

async def worker():
    await abide(evt.wait)
    print('Working')
    ...

def main():
    ...
    evt.set()
    ...

For condition variables and reentrant locks, you should use reserve_thread=True keyword argument to make sure the same thread is used throughout the block. For example:

import curio
import threading
import collections

# A thread
def producer(cond, items):
    for n in range(10):
        with cond:
             items.append(n)
             cond.notify()
    print('Producer done')

# A curio task
async def consumer(cond, items):
    while True:
        async with abide(cond, reserve_thread=True) as c:
            while not items:
                await c.wait()
            item = items.popleft()
            if item is None:
                break
            print('Consumer got:', item)
    print('Consumer done')

cond = threading.Condition()
items = collections.deque()

threading.Thread(target=producer, args=(cond, items)).start()
curio.run(consumer(cond, items))

A notable feature of abide() is that it also accepts Curio’s own synchronization primitives. Thus, you can write code that works independently of the lock type. For example, the first locking example could be rewritten as follows and the child would still work:

import curio

# A curio task (works with any lock)
async def child(lock):
    async with curio.abide(lock):
        print('Child has the lock')

# Another curio task
async def main():
    lock = curio.Lock()
    async with lock:
         print('Parent has the lock')
         await curio.spawn(child(lock))
         await curio.sleep(5)

curio.run(main())

A special circle of hell awaits code that combines the use of the abide() function with task cancellation. Although cancellation is mostly supported, there are a few things to keep in mind about it. First, if you are using abide(func, arg1, arg2, ...) to run a synchronous function, that function will fully run to completion in a separate thread regardless of the cancellation. So, if there are any side-effects associated with that code executing, you’ll need to take them into account. Second, if you are using async with abide(lock) with a thread-lock and a cancellation request is received while waiting for the lock.__enter__() method to execute, a background thread continues to run waiting for the eventual lock acquisition. Once acquired, curio releases it again. However, fully figuring out what’s happening might be mind-bending.

The abide() function can be used to synchronize with a thread reentrant lock (e.g., threading.RLock). However, reentrancy is not supported. Each lock acquisition using abide() involves a different backing thread. Repeated acquisitions would violate the constraint that reentrant locks have on only being acquired by a single thread.

All things considered, it’s probably best to try and avoid code that synchronizes Curio tasks with threads. However, if you must, Curio abides.

Asynchronous Threads

If you need to perform a lot of synchronous operations, but still interact with Curio, you might consider launching an asynchronous thread. An asynchronous thread flips the whole world around–instead of executing synchronous operations using run_in_thread(), you kick everything out to a thread and perform the asynchronous operations using a magic AWAIT() function.

class AsyncThread(target, args=(), kwargs={}, daemon=True)

Creates an asynchronous thread. The arguments are the same as for the threading.Thread class. target is a synchronous callable. args and kwargs are its arguments. daemon specifies if the thread runs in daemonic mode.

await AsyncThread.start()

Starts the asynchronous thread.

await join()

Waits for the thread to terminate, returning the callables final result. The final result is returned in the same manner as the usual Task.join() method used on Curio tasks.

await cancel()

Cancels the asynchronous thread. The behavior is the same as cancellation performed on Curio tasks. An asynchronous thread can only be cancelled when it performs blocking operations on asynchronous objects (e.g., using AWAIT().

Within a thread, the following function can be used to execute a coroutine.

AWAIT(coro)

Execute a coroutine on behalf of an asynchronous thread. The requested coroutine executes in Curio’s main execution thread. The caller is blocked until it completes. If used outside of an asynchronous thread, an AsyncOnlyError exception is raised. If coro is not a coroutine, it is returned unmodified. The reason AWAIT is all-caps is to make it more easily heard when there are all of these coders yelling at you to just use pure async code instead of launching a thread. Also, await is likely to be a reserved keyword in Python 3.7.

Here is a simple example of an asynchronous thread that reads data off a Curio queue:

from curio import run, Queue, sleep, CancelledError
from curio.thread import AsyncThread, AWAIT

def consumer(queue):
    try:
        while True:
            item = AWAIT(queue.get())
            print('Got:', item)
            AWAIT(queue.task_done())

    except CancelledError:
        print('Consumer goodbye!')
        raise

async def main():
    q = Queue()
    t = AsyncThread(target=consumer, args=(q,))
    await t.start()

    for i in range(10):
        await q.put(i)
        await sleep(1)

    await q.join()
    await t.cancel()

run(main())

Asynchronous threads can also be created using the following decorator.

async_thread(callable)

A decorator that adapts a synchronous callable into an asynchronous function that runs an asynchronous thread.

Using this decorator, you can write a function like this:

@async_thread
def consumer(queue):
    try:
        while True:
            item = AWAIT(queue.get())
            print('Got:', item)
            AWAIT(queue.task_done())

    except CancelledError:
        print('Consumer goodbye!')
        raise

Now, whenever the code executes (e.g., await consumer(q)), a thread will automatically be created. The decorator might also be useful in combination with spawn() like this:

def consumer(queue):
    try:
        while True:
            item = AWAIT(queue.get())
            print('Got:', item)
            AWAIT(queue.task_done())

    except CancelledError:
        print('Consumer goodbye!')
        raise

async def main():
    q = Queue()
    t = await spawn(async_thread(consumer)(q))
    ...

Asynchronous threads can use all of Curio’s features including coroutines, asynchronous context managers, asynchronous iterators, timeouts and more. For coroutines, use the AWAIT() function. For context managers and iterators, use the synchronous counterpart. For example, you could write this:

from curio.thread import async_thread, AWAIT
from curio import run, tcp_server

@async_thread
def echo_client(client, addr):
    print('Connection from:', addr)
    with client:
        f = client.as_stream()
        for line in f:
            AWAIT(client.sendall(line))
    print('Client goodbye')

run(tcp_server('', 25000, echo_client))

In this code, the with client and for line in f statements are actually executing asynchronous code behind the scenes.

Asynchronous threads can perform any combination of blocking operations including those that might involve normal thread-related primitives such as locks and queues. These operations will block the thread itself, but will not block the Curio kernel loop. In a sense, this is the whole point–if you run things in an async threads, the rest of Curio is protected. Asynchronous threads can be cancelled in the same manner as normal Curio tasks. However, the same rules apply–an asynchronous thread can only be cancelled on blocking operations involving AWAIT().

A final curious thing about async threads is that the AWAIT() function is no-op if you don’t give it a coroutine. This means that code, in many cases, can be made to be compatible with regular Python threads. For example, this code actually runs:

from curio.thread import AWAIT
from curio import CancelledError
from threading import Thread
from queue import Queue
from time import sleep

def consumer(queue):
    try:
        while True:
            item = AWAIT(queue.get())
            print('Got:', item)
            AWAIT(queue.task_done())

    except CancelledError:
        print('Consumer goodbye!')
        raise

def main():
    q = Queue()
    t = Thread(target=consumer, args=(q,), daemon=True)
    t.start()

    for i in range(10):
        q.put(i)
        sleep(1)
    q.join()

main()

In this code, consumer() is simply launched in a regular thread with a regular thread queue. The AWAIT() operations do nothing–the queue operations aren’t coroutines and their results return unmodified. Certain Curio features such as cancellation aren’t supported by normal threads so that would be ignored. However, it’s interesting that you can write a kind of hybrid code that works in both a threaded and asynchronous world.

Signals

Unix signals are managed by the SignalSet class. This class operates as an asynchronous context manager. The recommended usage looks like this:

import signal

async def coro():
    ...
    async with SignalSet(signal.SIGUSR1, signal.SIGHUP) as sigset:
          ...
          signo = await sigset.wait()
          print('Got signal', signo)
          ...

For all of the statements inside the context-manager, signals will be queued. The sigset.wait() operation will return received signals one at a time from the signal queue.

Signals can be temporarily ignored using a normal context manager:

async def coro():
    ...
    sigset = SignalSet(signal.SIGINT)
    with sigset.ignore():
          ...
          # Signals temporarily disabled
          ...

Caution: Signal handling only works if the curio kernel is running in Python’s main execution thread. Also, mixing signals with threads, subprocesses, and other concurrency primitives is a well-known way to make your head shatter into small pieces. Tread lightly.

class SignalSet(*signals)

Represents a set of one or more Unix signals. signals is a list of signals as defined in the built-in signal module.

The following methods are available on a SignalSet instance. They may only be used in coroutines.

await SignalSet.wait()

Wait for one of the signals in the signal set to arrive. Returns the signal number of the signal received. Normally this method is used inside an async with statement because this allows received signals to be properly queued. It can be used in isolation, but be aware that this will only catch a single signal right at that line of code. It’s possible that you might lose signals if you use this method outside of a context manager.

SignalSet.ignore()

Returns a context manager wherein signals from the signal set are temporarily disabled. Note: This is a normal context manager– use a normal with-statement.

Asynchronous Metaprogramming

The curio.meta module provides some decorators and metaclasses that might be useful if writing larger programs involving coroutines.

class curio.meta.AsyncABC

A base class that provides the functionality of a normal abstract base class, but additionally enforces coroutine-correctness on methods in subclasses. That is, if a method is defined as a coroutine in a parent class, then it must also be a coroutine in child classes.

Here is an example:

from curio.abc import AsyncABC, abstractmethod

class Base(AsyncABC):
    @abstractmethod
    async def spam(self):
        pass

    @abstractmethod
    async def grok(self):
        pass

class Child(Base):
    async def spam(self):
        pass

c = Child()   # Error -> grok() not defined

class Child2(Base):
    def spam(self):     # Error -> Not defined using async def
        pass

    async def grok(self):
        pass

The enforcement of coroutines is applied to all methods. Thus, the following classes would also generate an error:

class Base(AsyncABC):
    async def spam(self):
        pass

    async def grok(self):
        pass

class Child(Base):
    def spam(self):     # Error -> Not defined using async def
        pass

class curio.meta.AsyncObject

A base class that provides all of the functionality of AsyncABC, but additionally requires instances to be created inside of coroutines. The __init__() method must be defined as a coroutine and may call other coroutines.

Here is an example using AsyncObject:

from curio.meta import AsyncObject

class Spam(AsyncObject):
    async def __init__(self, x, y):
        self.x = x
        self.y = y

# To create an instance
async def func():
    s = await Spam(2, 3)
    ...

curio.meta.blocking(func)

A decorator that indicates that the function performs a blocking operation. If the function is called from within a coroutine, the function is executed in a separate thread and await is used to obtain the result. If the function is called from normal synchronous code, then the function executes normally. The Curio run_in_thread() coroutine is used to execute the function in a thread.

curio.meta.cpubound(func)

A decorator that indicates that the function performs CPU intensive work. If the function is called from within a coroutine, the function is executed in a separate process and await is used to obtain the result. If the function is called from normal synchronous code, then the function executes normally. The Curio run_in_process() coroutine is used to execute the function in a process.

The @blocking and @cpubound decorators are interesting in that they make normal Python functions usable from both asynchronous and synchronous code. For example, consider this example:

import curio
from curio.meta import blocking
import time

@blocking
def slow(name):
    time.sleep(30)
    return 'Hello ' + name

async def main():
    result = await slow('Dave')      # Async execution
    print(result)

if __name__ == '__main__':
    result = slow('Guido')           # Sync execution
    print(result)
    curio.run(main())

In this example, the slow() function can be used from both coroutines and normal synchronous code. However, when called in a coroutine, await must be used. Behind the scenes, the function runs in a thread–preventing the function from blocking the execution of other coroutines.

curio.meta.awaitable(syncfunc)

A decorator that allows an asynchronous implementation of a function to be attached to an existing synchronous function. If the resulting function is called from synchronous code, the synchronous function is used. If the function is called from asynchronous code, the asynchronous function is used.

Here is an example that illustrates:

import curio
from curio.meta import awaitable

def spam(x, y):
    print('Synchronous ->', x, y)

@awaitable(spam)
async def spam(x, y):
    print('Asynchronous ->', x, y)

async def main():
    await spam(2, 3)        # Calls asynchronous spam()

if __name__ == '__main__':
   spam(2, 3)               # Calls synchronous spam()
   curio.run(main())

Exceptions

The following exceptions are defined. All are subclasses of the CurioError base class.

exception curio.CurioError

Base class for all Curio-specific exceptions.

exception curio.CancelledError

Base class for all cancellation-related exceptions.

exception curio.TaskCancelled

Exception raised in a coroutine if it has been cancelled using the Task.cancel() method. If ignored, the coroutine is silently terminated. If caught, a coroutine can continue to run, but should work to terminate execution. Ignoring a cancellation request and continuing to execute will likely cause some other task to hang.

exception curio.TaskTimeout

Exception raised in a coroutine if it has been cancelled by timeout.

exception curio.TimeoutCancellationError

Exception raised in a coroutine if it has been cancelled due to a timeout, but not one related to the inner-most timeout operation.

exception curio.TaskError

Exception raised by the Task.join() method if an uncaught exception occurs in a task. It is a chained exception. The __cause__ attribute contains the exception that causes the task to fail.

Low-level Kernel System Calls

The following system calls are available, but not typically used directly in user code. They are used to implement higher level objects such as locks, socket wrappers, and so forth. If you find yourself using these, you’re probably doing something wrong–or implementing a new curio primitive. These calls are found in the curio.traps submodule.

Traps come in two flavors: blocking and synchronous. A blocking trap might block for an indefinite period of time while allowing other tasks to run, and always checks for and raises any pending timeouts or cancellations. A synchronous trap is implemented by trapping into the kernel, but semantically it acts like a regular synchronous function call. Specifically, this means that it always returns immediately without running any other task, and that it does not act as a cancellation point.

await curio.traps._read_wait(fileobj)

Blocking trap. Sleep until data is available for reading on fileobj. fileobj is any file-like object with a fileno() method.

await curio.traps._write_wait(fileobj)

Blocking trap. Sleep until data can be written on fileobj. fileobj is any file-like object with a fileno() method.

await curio.traps._future_wait(future)

Blocking trap. Sleep until a result is set on future. future is an instance of concurrent.futures.Future.

await curio.traps._join_task(task)

Blocking trap. Sleep until the indicated task completes. After this trap completes, then the task’s return value or raised exception information is available in task.next_value or task.exc_info, respectively.

await curio.traps._cancel_task(task)

Synchronous trap. Cancel the indicated task.

await curio.traps._wait_on_ksync(ksync, state_name)

Blocking trap. Go to sleep on a kernel synchronization primitive. ksync is an instance of curio.kernel.KernelSyncBase. state_name is the name of the wait state (used in debugging).

await curio.traps._reschedule_tasks(ksync, n=1, value=None, exc=None)

Synchronous trap. Reschedule one or more tasks from a kernel synchronization primitive. n is the number of tasks to release. value and exc specify the return value or exception to raise in the task when it resumes execution.

await curio.traps._sigwatch(sigset)

Synchronous trap. Tell the kernel to start queuing signals in the given signal set sigset.

await curio.traps._sigunwatch(sigset)

Synchronous trap. Tell the kernel to stop queuing signals in the given signal set.

await curio.traps._sigwait(sigset)

Blocking trap. Wait for the arrival of a signal in a given signal set. Returns the signal number of the received signal.

await curio.traps._get_kernel()

Synchronous trap. Get a reference to the running Kernel object.

await curio.traps._get_current()

Synchronous trap. Get a reference to the currently running Task instance.

await curio.traps._set_timeout(seconds)

Synchronous trap. Set a timeout in the currently running task. Returns the previous timeout (if any)

await curio.traps._unset_timeout(previous)

Synchronous trap. Unset a timeout in the currently running task. previous is the value returned by the _set_timeout() call used to set the timeout.

await curio.traps._ksync_reschedule_function(queue)

Synchronous trap. Return a function that allows tasks to be rescheduled on a kernel sychronization primitive without the use of await. Can be used in synchronous code as long as it runs in the same thread as the Curio kernel.

_clock():

Synchronous trap. Returns the current time according to the Curio kernel’s clock.

Again, you’re unlikely to use any of these functions directly. However, here’s a small taste of how they’re used. For example, the curio.io.Socket.recv() method looks roughly like this:

class Socket(object):
    ...
    def recv(self, maxbytes):
        while True:
            try:
                return self._socket.recv(maxbytes)
            except BlockingIOError:
                await _read_wait(self._socket)
    ...

This method first tries to receive data. If none is available, the _read_wait() call is used to put the task to sleep until reading can be performed. When it awakes, the receive operation is retried. Just to emphasize, the _read_wait() doesn’t actually perform any I/O. It’s just scheduling a task for it.