Platform Support

The 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

• loop.add_reader() and loop.add_writer() cannot be used to monitor file I/O.

Windows

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

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Changed in version 3.8: On Windows, ProactorEventLoop is now the default event loop.

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

• loop.create_unix_connection() and loop.create_unix_server() are not supported. The socket.AF_UNIX socket family is specific to Unix.
• loop.add_signal_handler() and loop.remove_signal_handler() are not supported.

SelectorEventLoop has the following limitations:

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

ProactorEventLoop has the following limitations:

• The loop.add_reader() and loop.add_writer() methods 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.

Subprocess Support on Windows

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

The policy.set_child_watcher() function is also not supported, as ProactorEventLoop has a different mechanism to watch child processes.

macOS

Modern macOS versions are fully supported.

macOS <= 10.8

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