.. currentmodule:: asyncio
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.
- :meth:`loop.add_reader` and :meth:`loop.add_writer` cannot be used to monitor file I/O.
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) and on the Windows configuration.
On Windows, the default event loop :class:`ProactorEventLoop` supports subprocesses, whereas :class:`SelectorEventLoop` does not.
Modern macOS versions are fully supported.
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)