18.5.1. Base Event Loop

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

Base class of event loops.

This class is not thread safe.

18.5.1.1. Run an event loop

BaseEventLoop.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.

BaseEventLoop.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.

BaseEventLoop.is_running()

Returns running status of event loop.

BaseEventLoop.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.

BaseEventLoop.is_closed()

Returns True if the event loop was closed.

New in version 3.4.2.

BaseEventLoop.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.

18.5.1.2. 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.

BaseEventLoop.call_soon(callback, *args)

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

Use functools.partial to pass keywords to the callback.

BaseEventLoop.call_soon_threadsafe(callback, *args)

Like call_soon(), but thread safe.

See the concurrency and multithreading section of the documentation.

18.5.1.3. 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.

BaseEventLoop.call_later(delay, callback, *args)

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

An instance of asyncio.Handle 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().

Use functools.partial to pass keywords to the callback.

BaseEventLoop.call_at(when, callback, *args)

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

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

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

Use functools.partial to pass keywords to the callback.

BaseEventLoop.time()

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

See also

The asyncio.sleep() function.

18.5.1.4. Tasks

BaseEventLoop.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.

This method was added in Python 3.4.2. Use the async() function to support also older Python versions.

New in version 3.4.2.

BaseEventLoop.set_task_factory(factory)

Set a task factory that will be used by BaseEventLoop.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.

BaseEventLoop.get_task_factory()

Return a task factory, or None if the default one is in use.

New in version 3.4.4.

18.5.1.5. Creating connections

18.5.1.6. Creating listening connections

18.5.1.7. 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.

BaseEventLoop.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.

BaseEventLoop.remove_reader(fd)

Stop watching the file descriptor for read availability.

BaseEventLoop.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.

BaseEventLoop.remove_writer(fd)

Stop watching the file descriptor for write availability.

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

18.5.1.8. Low-level socket operations

18.5.1.9. Resolve host name

18.5.1.10. Connect pipes

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

See also

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

18.5.1.11. UNIX signals

Availability: UNIX only.

BaseEventLoop.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.

BaseEventLoop.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.

18.5.1.12. 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).

BaseEventLoop.set_default_executor(executor)

Set the default executor used by run_in_executor().

18.5.1.13. Error Handling API

Allows to customize how exceptions are handled in the event loop.

BaseEventLoop.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).

BaseEventLoop.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().

BaseEventLoop.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.

18.5.1.14. Debug mode

BaseEventLoop.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.

BaseEventLoop.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.

18.5.1.15. Server

class asyncio.Server

Server listening on sockets.

Object created by the BaseEventLoop.create_server() method and the start_server() function. Don’t instantiate the class directly.

close()

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

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

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

sockets

List of socket.socket objects the server is listening to, or None if the server is closed.

18.5.1.16. Handle

class asyncio.Handle

A callback wrapper object returned by BaseEventLoop.call_soon(), BaseEventLoop.call_soon_threadsafe(), BaseEventLoop.call_later(), and BaseEventLoop.call_at().

cancel()

Cancel the call. If the callback is already canceled or executed, this method has no effect.

18.5.1.17. Event loop examples

18.5.1.17.1. Hello World with call_soon()

Example using the BaseEventLoop.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.

18.5.1.17.2. Display the current date with call_later()

Example of callback displaying the current date every second. The callback uses the BaseEventLoop.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.

18.5.1.17.3. Watch a file descriptor for read events

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

import asyncio
try:
    from socket import socketpair
except ImportError:
    from asyncio.windows_utils 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 BaseEventLoop.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.

18.5.1.17.4. Set signal handlers for SIGINT and SIGTERM

Register handlers for signals SIGINT and SIGTERM using the BaseEventLoop.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.