asyncio-platforms

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.
• loop.connect_read_pipe and loop.connect_write_pipe cannot be used with regular files. See Supported pipe objects <asyncio-pipe-objects> for the objects that are accepted on each platform.

Windows

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

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:

• ~selectors.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 <asyncio-subprocess> 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.
• loop.connect_read_pipe and loop.connect_write_pipe only accept a handle opened for overlapped I/O. See Supported pipe objects <asyncio-pipe-objects> for which objects are 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) and on the Windows configuration.

Subprocess Support on Windows

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

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 ~selectors.SelectSelector or ~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)