Event Loop

Preface

The event loop is the core of every asyncio application. Event loops run asynchronous tasks and callbacks, perform network IO operations, and run subprocesses.

Application developers should typically use the high-level asyncio functions, such as asyncio.run(), and should rarely need to reference the loop object or call its methods. This section is intended mostly for authors of lower-level code, libraries, and frameworks, who need finer control over the event loop behavior.

Obtaining the Event Loop

The following low-level functions can be used to get, set, or create an event loop:

asyncio.get_running_loop()

Return the running event loop in the current OS thread.

If there is no running event loop a RuntimeError is raised. This function can only be called from a coroutine or a callback.

New in version 3.7.

asyncio.get_event_loop()

Get the current event loop.

If there is no current event loop set in the current OS thread, the OS thread is main, and set_event_loop() has not yet been called, asyncio will create a new event loop and set it as the current one.

Because this function has rather complex behavior (especially when custom event loop policies are in use), using the get_running_loop() function is preferred to get_event_loop() in coroutines and callbacks.

Consider also using the asyncio.run() function instead of using lower level functions to manually create and close an event loop.

asyncio.set_event_loop(loop): Set loop as a current event loop for the current OS thread.

asyncio.new_event_loop(): Create a new event loop object.

Note that the behaviour of get_event_loop(), set_event_loop(), and new_event_loop() functions can be altered by setting a custom event loop policy.

Contents

This documentation page contains the following sections:

The Event Loop Methods section is the reference documentation of the event loop APIs;
The Callback Handles section documents the Handle and TimerHandle instances which are returned from scheduling methods such as loop.call_soon() and loop.call_later();
The Server Objects section documents types returned from event loop methods like loop.create_server();
The Event Loop Implementations section documents the SelectorEventLoop and ProactorEventLoop classes;
The Examples section showcases how to work with some event loop APIs.

Event Loop Methods

Event loops have low-level APIs for the following:

Running and stopping the loop
Scheduling callbacks
Scheduling delayed callbacks
Creating Futures and Tasks
Opening network connections
Creating network servers
Transferring files
TLS Upgrade
Watching file descriptors
Working with socket objects directly
DNS
Working with pipes
Unix signals
Executing code in thread or process pools
Error Handling API
Enabling debug mode
Running Subprocesses

Running and stopping the loop

loop.run_until_complete(future)

Run until the future (an instance of Future) has completed.

If the argument is a coroutine object it is implicitly scheduled to run as a asyncio.Task.

Return the Future’s result or raise its exception.

loop.run_forever()

Run the event loop until stop() is called.

If stop() is called before run_forever() is called, the loop will poll the I/O selector once with a timeout of zero, run all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exit.

If stop() is called while run_forever() is running, the loop will run the current batch of callbacks and then exit. Note that new callbacks scheduled by callbacks will not run in this case; instead, they will run the next time run_forever() or run_until_complete() is called.

loop.stop(): Stop the event loop.

loop.is_running(): Return True if the event loop is currently running.

loop.is_closed(): Return True if the event loop was closed.

loop.close()

Close the event loop.

The loop must not be running when this function is called. Any pending callbacks will be discarded.

This method clears all queues and shuts down the executor, but does not wait for the executor to finish.

This method is idempotent and irreversible. No other methods should be called after the event loop is closed.

Scheduling callbacks

loop.call_soon(callback, *args, context=None)

Schedule a callback to be called with args arguments at the next iteration of the event loop.

Callbacks are called in the order in which they are registered. Each callback will be called exactly once.

An optional keyword-only context argument allows specifying a custom contextvars.Context for the callback to run in. The current context is used when no context is provided.

An instance of asyncio.Handle is returned, which can be used later to cancel the callback.

This method is not thread-safe.

loop.call_soon_threadsafe(callback, *args, context=None)

A thread-safe variant of call_soon(). Must be used to schedule callbacks from another thread.

See the concurrency and multithreading section of the documentation.

Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

Note

Most asyncio scheduling functions don’t allow passing keyword arguments. To do that, use functools.partial():

# will schedule "print("Hello", flush=True)"
loop.call_soon(
    functools.partial(print, "Hello", flush=True))

Using partial objects is usually more convenient than using lambdas, as asyncio can render partial objects better in debug and error messages.

Scheduling delayed callbacks

Event loop provides mechanisms to schedule callback functions to be called at some point in the future. Event loop uses monotonic clocks to track time.

loop.call_later(delay, callback, *args, context=None)

Schedule callback to be called after the given delay number of seconds (can be either an int or a float).

An instance of asyncio.TimerHandle is returned which can be used to cancel the callback.

callback will be called exactly once. If two callbacks are scheduled for exactly the same time, the order in which they are called is undefined.

The optional positional args will be passed to the callback when it is called. If you want the callback to be called with keyword arguments use functools.partial().

An optional keyword-only context argument allows specifying a custom contextvars.Context for the callback to run in. The current context is used when no context is provided.

Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

Changed in version 3.7.1: In Python 3.7.0 and earlier with the default event loop implementation, the delay could not exceed one day. This has been fixed in Python 3.7.1.

loop.call_at(when, callback, *args, context=None)

Schedule callback to be called at the given absolute timestamp when (an int or a float), using the same time reference as loop.time().

This method’s behavior is the same as call_later().

An instance of asyncio.TimerHandle is returned which can be used to cancel the callback.

Changed in version 3.7: The context keyword-only parameter was added. See PEP 567 for more details.

Changed in version 3.7.1: In Python 3.7.0 and earlier with the default event loop implementation, the difference between when and the current time could not exceed one day. This has been fixed in Python 3.7.1.

loop.time(): Return the current time, as a float value, according to the event loop’s internal monotonic clock.

Note

Changed in version 3.8: In Python 3.7 and earlier timeouts (relative delay or absolute when) should not exceed one day. This has been fixed in Python 3.8.

See also

The asyncio.sleep() function.

Creating Futures and Tasks

loop.create_future()

Create an asyncio.Future object attached to the event loop.

This is the preferred way to create Futures in asyncio. This lets third-party event loops provide alternative implementations of the Future object (with better performance or instrumentation).

New in version 3.5.2.

loop.create_task(coro)

Schedule the execution of a Coroutines. Return a Task object.

Third-party event loops can use their own subclass of Task for interoperability. In this case, the result type is a subclass of Task.

loop.set_task_factory(factory)

Set a task factory that will be used by loop.create_task().

If factory is None the default task factory will be set. Otherwise, factory must be a callable with the signature matching (loop, coro), where loop is a reference to the active event loop, and coro is a coroutine object. The callable must return a asyncio.Future-compatible object.

loop.get_task_factory(): Return a task factory or None if the default one is in use.

Opening network connections

Creating network servers

Transferring files

TLS Upgrade

Watching file descriptors

loop.add_reader(fd, callback, \*args): Start monitoring the fd file descriptor for read availability and invoke callback with the specified arguments once fd is available for reading.

loop.remove_reader(fd): Stop monitoring the fd file descriptor for read availability.

loop.add_writer(fd, callback, \*args)

Start monitoring the fd file descriptor for write availability and invoke callback with the specified arguments once fd is available for writing.

Use functools.partial() to pass keyword arguments to callback.

loop.remove_writer(fd): Stop monitoring the fd file descriptor for write availability.

See also Platform Support section for some limitations of these methods.

Working with socket objects directly

In general, protocol implementations that use transport-based APIs such as loop.create_connection() and loop.create_server() are faster than implementations that work with sockets directly. However, there are some use cases when performance is not critical, and working with socket objects directly is more convenient.

DNS

Changed in version 3.7: Both getaddrinfo and getnameinfo methods were always documented to return a coroutine, but prior to Python 3.7 they were, in fact, returning asyncio.Future objects. Starting with Python 3.7 both methods are coroutines.

Working with pipes

Note

SelectorEventLoop does not support the above methods on Windows. Use ProactorEventLoop instead for Windows.

See also

The loop.subprocess_exec() and loop.subprocess_shell() methods.

Unix signals

loop.add_signal_handler(signum, callback, \*args)

Set callback as the handler for the signum signal.

The callback will be invoked by loop, along with other queued callbacks and runnable coroutines of that event loop. Unlike signal handlers registered using signal.signal(), a callback registered with this function is allowed to interact with the event loop.

Raise ValueError if the signal number is invalid or uncatchable. Raise RuntimeError if there is a problem setting up the handler.

Use functools.partial() to pass keyword arguments to callback.

Like signal.signal(), this function must be invoked in the main thread.

loop.remove_signal_handler(sig)

Remove the handler for the sig signal.

Return True if the signal handler was removed, or False if no handler was set for the given signal.

See also

The signal module.

Executing code in thread or process pools

loop.set_default_executor(executor)

Set executor as the default executor used by run_in_executor(). executor should be an instance of ThreadPoolExecutor.

Deprecated since version 3.7: Using an executor that is not an instance of ThreadPoolExecutor is deprecated and will trigger an error in Python 3.9.

executor must be an instance of concurrent.futures.ThreadPoolExecutor.

Error Handling API

Allows customizing how exceptions are handled in the event loop.

loop.set_exception_handler(handler)

Set handler as the new event loop exception handler.

If handler is None, the default exception handler will be set. Otherwise, handler must be a callable with the signature matching (loop, context), where loop is a reference to the active event loop, and context is a dict object containing the details of the exception (see call_exception_handler() documentation for details about context).

loop.get_exception_handler(): Return the current exception handler, or None if no custom exception handler was set.

New in version 3.5.2.

loop.default_exception_handler(context)

Default exception handler.

This is called when an exception occurs and no exception handler is set. This can be called by a custom exception handler that wants to defer to the default handler behavior.

context parameter has the same meaning as in call_exception_handler().

loop.call_exception_handler(context)

Call the current event loop exception handler.

context is a dict object containing the following keys (new keys may be introduced in future Python versions):

‘message’: Error message;
‘exception’ (optional): Exception object;
‘future’ (optional): asyncio.Future instance;
‘handle’ (optional): asyncio.Handle instance;
‘protocol’ (optional): Protocol instance;
‘transport’ (optional): Transport instance;
‘socket’ (optional): socket.socket instance.

Note

This method should not be overloaded in subclassed event loops. For custom exception handling, use the set_exception_handler() method.

Enabling debug mode

loop.get_debug()

Get the debug mode (bool) of the event loop.

The default value is True if the environment variable PYTHONASYNCIODEBUG is set to a non-empty string, False otherwise.

loop.set_debug(enabled: bool): Set the debug mode of the event loop.

Changed in version 3.7: The new -X dev command line option can now also be used to enable the debug mode.

See also

The debug mode of asyncio.

Running Subprocesses

Methods described in this subsections are low-level. In regular async/await code consider using the high-level asyncio.create_subprocess_shell() and asyncio.create_subprocess_exec() convenience functions instead.

Note

The default asyncio event loop on Windows does not support subprocesses. See Subprocess Support on Windows for details.

Note

It is the application’s responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid shell injection vulnerabilities. The shlex.quote() function can be used to properly escape whitespace and special characters in strings that are going to be used to construct shell commands.

Callback Handles

class asyncio.Handle

A callback wrapper object returned by loop.call_soon(), loop.call_soon_threadsafe().

cancel(): Cancel the callback. If the callback has already been canceled or executed, this method has no effect.

cancelled(): Return True if the callback was cancelled.

New in version 3.7.

class asyncio.TimerHandle

A callback wrapper object returned by loop.call_later(), and loop.call_at().

This class is a subclass of Handle.

when()

Return a scheduled callback time as float seconds.

The time is an absolute timestamp, using the same time reference as loop.time().

New in version 3.7.

Server Objects

Server objects are created by loop.create_server(), loop.create_unix_server(), start_server(), and start_unix_server() functions.

Do not instantiate the class directly.

class asyncio.Server

Server objects are asynchronous context managers. When used in an async with statement, it’s guaranteed that the Server object is closed and not accepting new connections when the async with statement is completed:

srv = await loop.create_server(...)

async with srv:
    # some code

# At this point, srv is closed and no longer accepts new connections.

Changed in version 3.7: Server object is an asynchronous context manager since Python 3.7.

close()

Stop serving: close listening sockets and set the sockets attribute to None.

The sockets that represent existing incoming client connections are left open.

The server is closed asynchronously, use the wait_closed() coroutine to wait until the server is closed.

get_loop(): Return the event loop associated with the server object.

New in version 3.7.

is_serving(): Return True if the server is accepting new connections.

New in version 3.7.

sockets: List of socket.socket objects the server is listening on, or None if the server is closed.

Changed in version 3.7: Prior to Python 3.7 Server.sockets used to return an internal list of server sockets directly. In 3.7 a copy of that list is returned.

Event Loop Implementations

asyncio ships with two different event loop implementations: SelectorEventLoop and ProactorEventLoop.

By default asyncio is configured to use SelectorEventLoop on all platforms.

class asyncio.SelectorEventLoop

An event loop based on the selectors module.

Uses the most efficient selector available for the given platform. It is also possible to manually configure the exact selector implementation to be used:

import asyncio
import selectors

selector = selectors.SelectSelector()
loop = asyncio.SelectorEventLoop(selector)
asyncio.set_event_loop(loop)

class asyncio.ProactorEventLoop

An event loop for Windows that uses “I/O Completion Ports” (IOCP).

An example how to use ProactorEventLoop on Windows:

import asyncio
import sys

if sys.platform == 'win32':
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)

class asyncio.AbstractEventLoop

Abstract base class for asyncio-compliant event loops.

The Event Loop Methods section lists all methods that an alternative implementation of AbstractEventLoop should have defined.

Examples

Note that all examples in this section purposefully show how to use the low-level event loop APIs, such as loop.run_forever() and loop.call_soon(). Modern asyncio applications rarely need to be written this way; consider using the high-level functions like asyncio.run().

Hello World with call_soon()

An example using the loop.call_soon() method to schedule a callback. The callback displays "Hello World" and then stops the event loop:

import asyncio

def hello_world(loop):
    """A callback to print 'Hello World' and stop the event loop"""
    print('Hello World')
    loop.stop()

loop = asyncio.get_event_loop()

# Schedule a call to hello_world()
loop.call_soon(hello_world, loop)

# Blocking call interrupted by loop.stop()
try:
    loop.run_forever()
finally:
    loop.close()

See also

A similar Hello World example created with a coroutine and the run() function.

Display the current date with call_later()

An example of a callback displaying the current date every second. The callback uses the loop.call_later() method to reschedule itself after 5 seconds, and then stops the event loop:

import asyncio
import datetime

def display_date(end_time, loop):
    print(datetime.datetime.now())
    if (loop.time() + 1.0) < end_time:
        loop.call_later(1, display_date, end_time, loop)
    else:
        loop.stop()

loop = asyncio.get_event_loop()

# Schedule the first call to display_date()
end_time = loop.time() + 5.0
loop.call_soon(display_date, end_time, loop)

# Blocking call interrupted by loop.stop()
try:
    loop.run_forever()
finally:
    loop.close()

See also

A similar current date example created with a coroutine and the run() function.

Watch a file descriptor for read events

Wait until a file descriptor received some data using the loop.add_reader() method and then close the event loop:

import asyncio
from socket import socketpair

# Create a pair of connected file descriptors
rsock, wsock = socketpair()

loop = asyncio.get_event_loop()

def reader():
    data = rsock.recv(100)
    print("Received:", data.decode())

    # We are done: unregister the file descriptor
    loop.remove_reader(rsock)

    # Stop the event loop
    loop.stop()

# Register the file descriptor for read event
loop.add_reader(rsock, reader)

# Simulate the reception of data from the network
loop.call_soon(wsock.send, 'abc'.encode())

try:
    # Run the event loop
    loop.run_forever()
finally:
    # We are done. Close sockets and the event loop.
    rsock.close()
    wsock.close()
    loop.close()

See also

A similar example using transports, protocols, and the loop.create_connection() method.
Another similar example using the high-level asyncio.open_connection() function and streams.

Set signal handlers for SIGINT and SIGTERM

(This signals example only works on Unix.)

Register handlers for signals SIGINT and SIGTERM using the loop.add_signal_handler() method:

import asyncio
import functools
import os
import signal

def ask_exit(signame, loop):
    print("got signal %s: exit" % signame)
    loop.stop()

async def main():
    loop = asyncio.get_running_loop()

    for signame in {'SIGINT', 'SIGTERM'}:
        loop.add_signal_handler(
            getattr(signal, signame),
            functools.partial(ask_exit, signame, loop))

    await asyncio.sleep(3600)

print("Event loop running for 1 hour, press Ctrl+C to interrupt.")
print(f"pid {os.getpid()}: send SIGINT or SIGTERM to exit.")

asyncio.run(main())

Event Loop — Python documentation

Event Loop

Event Loop Methods

Running and stopping the loop

Scheduling callbacks

Scheduling delayed callbacks

Creating Futures and Tasks

Opening network connections

Creating network servers

Transferring files

TLS Upgrade

Watching file descriptors

Working with socket objects directly

DNS

Working with pipes

Unix signals

Executing code in thread or process pools

Error Handling API

Enabling debug mode

Running Subprocesses

Callback Handles

Server Objects

Event Loop Implementations

Examples

Hello World with call_soon()

Display the current date with call_later()

Watch a file descriptor for read events

Set signal handlers for SIGINT and SIGTERM