================ Platform Support

The :mod:asyncio module is designed to be portable, but some platforms have subtle differences and limitations due to the platforms' underlying architecture and capabilities.

All Platforms

:meth:loop.add_reader and :meth:loop.add_writer cannot be used to monitor file I/O.

Windows

Source code: :source:Lib/asyncio/proactor_events.py, :source:Lib/asyncio/windows_events.py, :source:Lib/asyncio/windows_utils.py

.. versionchanged:: 3.8

On Windows, :class:ProactorEventLoop is now the default event loop.

All event loops on Windows do not support the following methods:

:meth:loop.create_unix_connection and :meth:loop.create_unix_server are not supported. The :const:socket.AF_UNIX socket family is specific to Unix.
:meth:loop.add_signal_handler and :meth:loop.remove_signal_handler are not supported.

:class:SelectorEventLoop has the following limitations:

:class:~selectors.SelectSelector is used to wait on socket events: it supports sockets and is limited to 512 sockets.
:meth:loop.add_reader and :meth:loop.add_writer only accept socket handles (e.g. pipe file descriptors are not supported).
Pipes are not supported, so the :meth:loop.connect_read_pipe and :meth:loop.connect_write_pipe methods are not implemented.
:ref:Subprocesses <asyncio-subprocess> are not supported, i.e. :meth:loop.subprocess_exec and :meth:loop.subprocess_shell methods are not implemented.

:class:ProactorEventLoop has the following limitations:

The :meth:loop.add_reader and :meth:loop.add_writer methods are not supported.

The resolution of the monotonic clock on Windows is usually around 15.6 milliseconds. The best resolution is 0.5 milliseconds. The resolution depends on the hardware (availability of HPET <https://en.wikipedia.org/wiki/High_Precision_Event_Timer>_) and on the Windows configuration.

.. _asyncio-windows-subprocess:

Subprocess Support on Windows

On Windows, the default event loop :class:ProactorEventLoop supports subprocesses, whereas :class:SelectorEventLoop does not.

macOS

Modern macOS versions are fully supported.

.. rubric:: macOS <= 10.8

On macOS 10.6, 10.7 and 10.8, the default event loop uses :class:selectors.KqueueSelector, which does not support character devices on these versions. The :class:SelectorEventLoop can be manually configured to use :class:~selectors.SelectSelector or :class:~selectors.PollSelector to support character devices on these older versions of macOS. Example::

import asyncio
import selectors

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