Event loops

1. Event loop functions

The following functions are convenient shortcuts to accessing the methods of the global policy. Note that this provides access to the default policy, unless an alternative policy was set by calling set_event_loop_policy() earlier in the execution of the process.

asyncio.get_event_loop()

Equivalent to calling get_event_loop_policy().get_event_loop().

asyncio.set_event_loop(loop)

Equivalent to calling get_event_loop_policy().set_event_loop(loop).

asyncio.new_event_loop()

Equivalent to calling get_event_loop_policy().new_event_loop().

2. Available event loops

asyncio currently provides two implementations of event loops: SelectorEventLoop and ProactorEventLoop.

class asyncio.SelectorEventLoop

Event loop based on the selectors module. Subclass of AbstractEventLoop.

Use the most efficient selector available on the platform.

On Windows, only sockets are supported (ex: pipes are not supported): see the MSDN documentation of select.

class asyncio.ProactorEventLoop

Proactor event loop for Windows using “I/O Completion Ports” aka IOCP. Subclass of AbstractEventLoop.

Availability: Windows.

See also

MSDN documentation on I/O Completion Ports.

Example to use a ProactorEventLoop on Windows:

import asyncio, sys

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

3. Platform support

The asyncio module has been designed to be portable, but each platform still has subtle differences and may not support all asyncio features.

3.1. Windows

Common limits of Windows event loops:

• create_unix_connection() and create_unix_server() are not supported: the socket family socket.AF_UNIX is specific to UNIX
• add_signal_handler() and remove_signal_handler() are not supported
• EventLoopPolicy.set_child_watcher() is not supported. ProactorEventLoop supports subprocesses. It has only one implementation to watch child processes, there is no need to configure it.

SelectorEventLoop specific limits:

• SelectSelector is used which only supports sockets and is limited to 512 sockets.
• add_reader() and add_writer() only accept file descriptors of sockets
• Pipes are not supported (ex: connect_read_pipe(), connect_write_pipe())
• Subprocesses are not supported (ex: subprocess_exec(), subprocess_shell())

ProactorEventLoop specific limits:

• create_datagram_endpoint() (UDP) is not supported
• add_reader() and add_writer() are not supported

The resolution of the monotonic clock on Windows is usually around 15.6 msec. The best resolution is 0.5 msec. The resolution depends on the hardware (availability of HPET) and on the Windows configuration. See asyncio delayed calls.

3.2. Mac OS X

Character devices like PTY are only well supported since Mavericks (Mac OS 10.9). They are not supported at all on Mac OS 10.5 and older.

On Mac OS 10.6, 10.7 and 10.8, the default event loop is SelectorEventLoop which uses selectors.KqueueSelector. selectors.KqueueSelector does not support character devices on these versions. The SelectorEventLoop can be used with SelectSelector or PollSelector to support character devices on these versions of Mac OS X. Example:

import asyncio
import selectors

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

4. Event loop policies and the default policy

Event loop management is abstracted with a policy pattern, to provide maximal flexibility for custom platforms and frameworks. Throughout the execution of a process, a single global policy object manages the event loops available to the process based on the calling context. A policy is an object implementing the AbstractEventLoopPolicy interface.

For most users of asyncio, policies never have to be dealt with explicitly, since the default global policy is sufficient.

The default policy defines context as the current thread, and manages an event loop per thread that interacts with asyncio. The module-level functions get_event_loop() and set_event_loop() provide convenient access to event loops managed by the default policy.

5. Event loop policy interface

An event loop policy must implement the following interface:

class asyncio.AbstractEventLoopPolicy

Event loop policy.

get_event_loop()

Get the event loop for the current context.

Returns an event loop object implementing the AbstractEventLoop interface.

Raises an exception in case no event loop has been set for the current context and the current policy does not specify to create one. It must never return None.

set_event_loop(loop)

Set the event loop for the current context to loop.

new_event_loop()

Create and return a new event loop object according to this policy’s rules.

If there’s need to set this loop as the event loop for the current context, set_event_loop() must be called explicitly.

6. Access to the global loop policy

asyncio.get_event_loop_policy()

Get the current event loop policy.

asyncio.set_event_loop_policy(policy)

Set the current event loop policy. If policy is None, the default policy is restored.