Base Event Loop¶

Source code: :source:`Lib/asyncio/events.py`

The event loop is the central execution device provided by asyncio. It provides multiple facilities, including:

Registering, executing and cancelling delayed calls (timeouts).
Creating client and server transports for various kinds of communication.
Launching subprocesses and the associated transports for communication with an external program.
Delegating costly function calls to a pool of threads.

class asyncio.BaseEventLoop¶: This class is an implementation detail. It is a subclass of AbstractEventLoop and may be a base class of concrete event loop implementations found in asyncio. It should not be used directly; use AbstractEventLoop instead. BaseEventLoop should not be subclassed by third-party code; the internal interface is not stable.

class asyncio.AbstractEventLoop¶

Abstract base class of event loops.

This class is not thread safe.

Run an event loop¶

AbstractEventLoop.run_forever()¶: Run until stop() is called. If stop() is called before run_forever() is called, this polls the I/O selector once with a timeout of zero, runs all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exits. If stop() is called while run_forever() is running, this will run the current batch of callbacks and then exit. Note that callbacks scheduled by callbacks will not run in that case; they will run the next time run_forever() is called.

Changed in version 3.5.1.

AbstractEventLoop.run_until_complete(future)¶

Run until the Future is done.

If the argument is a coroutine object, it is wrapped by ensure_future().

Return the Future’s result, or raise its exception.

AbstractEventLoop.is_running()¶: Returns running status of event loop.

AbstractEventLoop.stop()¶

Stop running the event loop.

This causes run_forever() to exit at the next suitable opportunity (see there for more details).

Changed in version 3.5.1.

AbstractEventLoop.is_closed()¶: Returns True if the event loop was closed.

New in version 3.4.2.

AbstractEventLoop.close()¶

Close the event loop. The loop must not be running. Pending callbacks will be lost.

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

This is idempotent and irreversible. No other methods should be called after this one.

Calls¶

Most asyncio functions don’t accept keywords. If you want to pass keywords to your callback, use functools.partial(). For example, loop.call_soon(functools.partial(print, "Hello", flush=True)) will call print("Hello", flush=True).

Note

functools.partial() is better than lambda functions, because asyncio can inspect functools.partial() object to display parameters in debug mode, whereas lambda functions have a poor representation.

AbstractEventLoop.call_soon(callback, *args, context=None)¶

Arrange for a callback to be called as soon as possible. The callback is called after call_soon() returns, when control returns to the event loop.

This operates as a FIFO queue, callbacks are called in the order in which they are registered. Each callback will be called exactly once.

Any positional arguments after the callback will be passed to the callback when it is called.

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 to cancel the callback.

Use functools.partial to pass keywords to the callback.

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

AbstractEventLoop.call_soon_threadsafe(callback, *args, context=None)¶

Like call_soon(), but thread safe.

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.

Delayed calls¶

The event loop has its own internal clock for computing timeouts. Which clock is used depends on the (platform-specific) event loop implementation; ideally it is a monotonic clock. This will generally be a different clock than time.time().

Note

Timeouts (relative delay or absolute when) should not exceed one day.

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

Arrange for the callback to be called after the given delay seconds (either an int or float).

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

callback will be called exactly once per call to call_later(). If two callbacks are scheduled for exactly the same time, it is undefined which will be called first.

The optional positional args will be passed to the callback when it is called. If you want the callback to be called with some named arguments, use a closure or 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.

Use functools.partial to pass keywords to the callback.

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

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

Arrange for the callback to be called at the given absolute timestamp when (an int or float), using the same time reference as AbstractEventLoop.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.

Use functools.partial to pass keywords to the callback.

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

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

See also

The asyncio.sleep() function.

Futures¶

AbstractEventLoop.create_future()¶

Create an asyncio.Future object attached to the loop.

This is a preferred way to create futures in asyncio, as event loop implementations can provide alternative implementations of the Future class (with better performance or instrumentation).

New in version 3.5.2.

Tasks¶

AbstractEventLoop.create_task(coro)¶

Schedule the execution of a coroutine object: wrap it in a future. 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.

New in version 3.4.2.

AbstractEventLoop.set_task_factory(factory)¶

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

If factory is None the default task factory will be set.

If factory is a callable, it should have a signature matching (loop, coro), where loop will be a reference to the active event loop, coro will be a coroutine object. The callable must return an asyncio.Future compatible object.

New in version 3.4.4.

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

New in version 3.4.4.

Creating connections¶

Creating listening connections¶

File Transferring¶

TLS Upgrade¶

Watch file descriptors¶

On Windows with SelectorEventLoop, only socket handles are supported (ex: pipe file descriptors are not supported).

On Windows with ProactorEventLoop, these methods are not supported.

AbstractEventLoop.add_reader(fd, callback, *args)¶

Start watching the file descriptor for read availability and then call the callback with specified arguments.

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_reader(fd)¶: Stop watching the file descriptor for read availability.

AbstractEventLoop.add_writer(fd, callback, *args)¶

Start watching the file descriptor for write availability and then call the callback with specified arguments.

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_writer(fd)¶: Stop watching the file descriptor for write availability.

The watch a file descriptor for read events example uses the low-level AbstractEventLoop.add_reader() method to register the file descriptor of a socket.

Low-level socket operations¶

Resolve host name¶

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.

Connect pipes¶

On Windows with SelectorEventLoop, these methods are not supported. Use ProactorEventLoop to support pipes on Windows.

See also

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

UNIX signals¶

Availability: UNIX only.

AbstractEventLoop.add_signal_handler(signum, callback, *args)¶

Add a handler for a signal.

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 keywords to the callback.

AbstractEventLoop.remove_signal_handler(sig)¶

Remove a handler for a signal.

Return True if a signal handler was removed, False if not.

See also

The signal module.

Executor¶

Call a function in an Executor (pool of threads or pool of processes). By default, an event loop uses a thread pool executor (ThreadPoolExecutor).

AbstractEventLoop.run_in_executor(executor, func, *args)¶

Arrange for a func to be called in the specified executor.

The executor argument should be an Executor instance. The default executor is used if executor is None.

Use functools.partial to pass keywords to the *func*.

This method returns a asyncio.Future object.

Changed in version 3.5.3: BaseEventLoop.run_in_executor() no longer configures the max_workers of the thread pool executor it creates, instead leaving it up to the thread pool executor (ThreadPoolExecutor) to set the default.

AbstractEventLoop.set_default_executor(executor)¶: Set the default executor used by run_in_executor().

Error Handling API¶

Allows customizing how exceptions are handled in the event loop.

AbstractEventLoop.set_exception_handler(handler)¶

Set handler as the new event loop exception handler.

If handler is None, the default exception handler will be set.

If handler is a callable object, it should have a matching signature to (loop, context), where loop will be a reference to the active event loop, context will be a dict object (see call_exception_handler() documentation for details about context).

AbstractEventLoop.get_exception_handler()¶: Return the exception handler, or None if the default one is in use.

New in version 3.5.2.

AbstractEventLoop.default_exception_handler(context)¶

Default exception handler.

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

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

AbstractEventLoop.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 later):

‘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

Note: this method should not be overloaded in subclassed event loops. For any custom exception handling, use set_exception_handler() method.

Debug mode¶

AbstractEventLoop.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.

New in version 3.4.2.

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

New in version 3.4.2.

See also

The debug mode of asyncio.

Server¶

class asyncio.Server¶

Server listening on sockets.

Object created by AbstractEventLoop.create_server(), AbstractEventLoop.create_unix_server(), start_server(), and start_unix_server() functions. Don’t instantiate the class directly.

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()¶: Gives 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 to, or None if the server is closed.

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

Handle¶

class asyncio.Handle¶

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

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

cancelled()¶: Return True if the call was cancelled.

New in version 3.7.

class asyncio.TimerHandle¶

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

The class is inherited from Handle.

when()¶

Return a scheduled callback time as float seconds.

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

New in version 3.7.

SendfileNotAvailableError¶

exception asyncio.SendfileNotAvailableError¶

Sendfile syscall is not available, subclass of RuntimeError.

Raised if the OS does not support sendfile syscall for given socket or file type.

Event loop examples¶

Hello World with call_soon()¶

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

import asyncio

def hello_world(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()
loop.run_forever()
loop.close()

See also

The Hello World coroutine example uses a coroutine.

Display the current date with call_later()¶

Example of callback displaying the current date every second. The callback uses the AbstractEventLoop.call_later() method to reschedule itself during 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()
loop.run_forever()
loop.close()

See also

The coroutine displaying the current date example uses a coroutine.

Watch a file descriptor for read events¶

Wait until a file descriptor received some data using the AbstractEventLoop.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())

# Run the event loop
loop.run_forever()

# We are done, close sockets and the event loop
rsock.close()
wsock.close()
loop.close()

See also

The register an open socket to wait for data using a protocol example uses a low-level protocol created by the AbstractEventLoop.create_connection() method.

The register an open socket to wait for data using streams example uses high-level streams created by the open_connection() function in a coroutine.

Set signal handlers for SIGINT and SIGTERM¶

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

import asyncio
import functools
import os
import signal

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

loop = asyncio.get_event_loop()
for signame in ('SIGINT', 'SIGTERM'):
    loop.add_signal_handler(getattr(signal, signame),
                            functools.partial(ask_exit, signame))

print("Event loop running forever, press Ctrl+C to interrupt.")
print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
try:
    loop.run_forever()
finally:
    loop.close()

This example only works on UNIX.

Base Event Loop¶

Run an event loop¶

Calls¶

Delayed calls¶

Futures¶

Tasks¶

Creating connections¶

Creating listening connections¶

File Transferring¶

TLS Upgrade¶

Watch file descriptors¶

Low-level socket operations¶

Resolve host name¶

Connect pipes¶

UNIX signals¶

Executor¶

Error Handling API¶

Debug mode¶

Server¶

Handle¶

SendfileNotAvailableError¶

Event loop 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¶

asyncio developer notes

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