:mod:`selectors` -- High-level I/O multiplexing =============================================== .. module:: selectors :synopsis: High-level I/O multiplexing. .. versionadded:: 3.4 Introduction ------------ This module allows high-level and efficient I/O multiplexing, built upon the :mod:`select` module primitives. Users are encouraged to use this module instead, unless they want precise control over the OS-level primitives used. It defines a :class:`BaseSelector` abstract base class, along with several concrete implementations (:class:`KqueueSelector`, :class:`EpollSelector`...), that can be used to wait for I/O readiness notification on multiple file objects. In the following, "file object" refers to any object with a :meth:`fileno()` method, or a raw file descriptor. See :term:`file object`. :class:`DefaultSelector` is an alias to the most efficient implementation available on the current platform: this should be the default choice for most users. .. note:: The type of file objects supported depends on the platform: on Windows, sockets are supported, but not pipes, whereas on Unix, both are supported (some other types may be supported as well, such as fifos or special file devices). .. seealso:: :mod:`select` Low-level I/O multiplexing module. Classes ------- Classes hierarchy:: BaseSelector +-- SelectSelector +-- PollSelector +-- EpollSelector +-- KqueueSelector In the following, *events* is a bitwise mask indicating which I/O events should be waited for on a given file object. It can be a combination of the constants below: +-----------------------+-----------------------------------------------+ | Constant | Meaning | +=======================+===============================================+ | :const:`EVENT_READ` | Available for read | +-----------------------+-----------------------------------------------+ | :const:`EVENT_WRITE` | Available for write | +-----------------------+-----------------------------------------------+ .. class:: SelectorKey A :class:`SelectorKey` is a :class:`~collections.namedtuple` used to associate a file object to its underlying file decriptor, selected event mask and attached data. It is returned by several :class:`BaseSelector` methods. .. attribute:: fileobj File object registered. .. attribute:: fd Underlying file descriptor. .. attribute:: events Events that must be waited for on this file object. .. attribute:: data Optional opaque data associated to this file object: for example, this could be used to store a per-client session ID. .. class:: BaseSelector A :class:`BaseSelector` is used to wait for I/O event readiness on multiple file objects. It supports file stream registration, unregistration, and a method to wait for I/O events on those streams, with an optional timeout. It's an abstract base class, so cannot be instantiated. Use :class:`DefaultSelector` instead, or one of :class:`SelectSelector`, :class:`KqueueSelector` etc. if you want to specifically use an implementation, and your platform supports it. :class:`BaseSelector` and its concrete implementations support the :term:`context manager` protocol. .. attribute:: resolution Resolution of the selector in seconds. .. method:: register(fileobj, events, data=None) Register a file object for selection, monitoring it for I/O events. *fileobj* is the file object to monitor. It may either be an integer file descriptor or an object with a ``fileno()`` method. *events* is a bitwise mask of events to monitor. *data* is an opaque object. This returns a new :class:`SelectorKey` instance, or raises a :exc:`ValueError` in case of invalid event mask or file descriptor, or :exc:`KeyError` if the file object is already registered. .. method:: unregister(fileobj) Unregister a file object from selection, removing it from monitoring. A file object shall be unregistered prior to being closed. *fileobj* must be a file object previously registered. This returns the associated :class:`SelectorKey` instance, or raises a :exc:`KeyError` if *fileobj* is not registered. It will raise :exc:`ValueError` if *fileobj* is invalid (e.g. it has no ``fileno()`` method or its ``fileno()`` method has an invalid return value). .. method:: modify(fileobj, events, data=None) Change a registered file object's monitored events or attached data. This is equivalent to :meth:`BaseSelector.unregister(fileobj)` followed by :meth:`BaseSelector.register(fileobj, events, data)`, except that it can be implemented more efficiently. This returns a new :class:`SelectorKey` instance, or raises a :exc:`ValueError` in case of invalid event mask or file descriptor, or :exc:`KeyError` if the file object is not registered. .. method:: select(timeout=None) Wait until some registered file objects become ready, or the timeout expires. If ``timeout > 0``, this specifies the maximum wait time, in seconds. If ``timeout <= 0``, the call won't block, and will report the currently ready file objects. If *timeout* is ``None``, the call will block until a monitored file object becomes ready. This returns a list of ``(key, events)`` tuples, one for each ready file object. *key* is the :class:`SelectorKey` instance corresponding to a ready file object. *events* is a bitmask of events ready on this file object. .. note:: This method can return before any file object becomes ready or the timeout has elapsed if the current process receives a signal: in this case, an empty list will be returned. .. method:: close() Close the selector. This must be called to make sure that any underlying resource is freed. The selector shall not be used once it has been closed. .. method:: get_key(fileobj) Return the key associated with a registered file object. This returns the :class:`SelectorKey` instance associated to this file object, or raises :exc:`KeyError` if the file object is not registered. .. method:: get_map() Return a mapping of file objects to selector keys. This returns a :class:`~collections.abc.Mapping` instance mapping registered file objects to their associated :class:`SelectorKey` instance. .. class:: DefaultSelector() The default selector class, using the most efficient implementation available on the current platform. This should be the default choice for most users. .. class:: SelectSelector() :func:`select.select`-based selector. .. class:: PollSelector() :func:`select.poll`-based selector. .. class:: EpollSelector() :func:`select.epoll`-based selector. .. method:: fileno() This returns the file descriptor used by the underlying :func:`select.epoll` object. .. class:: KqueueSelector() :func:`select.kqueue`-based selector. .. method:: fileno() This returns the file descriptor used by the underlying :func:`select.kqueue` object. Examples -------- Here is a simple echo server implementation:: import selectors import socket sel = selectors.DefaultSelector() def accept(sock, mask): conn, addr = sock.accept() # Should be ready print('accepted', conn, 'from', addr) conn.setblocking(False) sel.register(conn, selectors.EVENT_READ, read) def read(conn, mask): data = conn.recv(1000) # Should be ready if data: print('echoing', repr(data), 'to', conn) conn.send(data) # Hope it won't block else: print('closing', conn) sel.unregister(conn) conn.close() sock = socket.socket() sock.bind(('localhost', 1234)) sock.listen(100) sock.setblocking(False) sel.register(sock, selectors.EVENT_READ, accept) while True: events = sel.select() for key, mask in events: callback = key.data callback(key.fileobj, mask)