19.4. "selectors" — High-level I/O multiplexing
***********************************************

New in version 3.4.

**Source code:** Lib/selectors.py

======================================================================


19.4.1. Introduction
====================

This module allows high-level and efficient I/O multiplexing, built
upon the "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 "BaseSelector" abstract base class, along with several
concrete implementations ("KqueueSelector", "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
"fileno()" method, or a raw file descriptor. See *file object*.

"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).

See also:

  "select"
     Low-level I/O multiplexing module.


19.4.2. Classes
===============

Classes hierarchy:

   BaseSelector
   +-- SelectSelector
   +-- PollSelector
   +-- EpollSelector
   +-- DevpollSelector
   +-- 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 modules constants below:

   +-------------------------+-------------------------------------------------+
   | Constant                | Meaning                                         |
   +=========================+=================================================+
   | "EVENT_READ"            | Available for read                              |
   +-------------------------+-------------------------------------------------+
   | "EVENT_WRITE"           | Available for write                             |
   +-------------------------+-------------------------------------------------+

class selectors.SelectorKey

   A "SelectorKey" is a "namedtuple" used to associate a file object
   to its underlying file descriptor, selected event mask and attached
   data. It is returned by several "BaseSelector" methods.

   fileobj

      File object registered.

   fd

      Underlying file descriptor.

   events

      Events that must be waited for on this file object.

   data

      Optional opaque data associated to this file object: for
      example, this could be used to store a per-client session ID.

class selectors.BaseSelector

   A "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 "DefaultSelector" instead, or one of
   "SelectSelector", "KqueueSelector" etc. if you want to specifically
   use an implementation, and your platform supports it.
   "BaseSelector" and its concrete implementations support the
   *context manager* protocol.

   abstractmethod 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 "SelectorKey" instance, or raises a
      "ValueError" in case of invalid event mask or file descriptor,
      or "KeyError" if the file object is already registered.

   abstractmethod 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 "SelectorKey" instance, or raises a
      "KeyError" if *fileobj* is not registered.  It will raise
      "ValueError" if *fileobj* is invalid (e.g. it has no "fileno()"
      method or its "fileno()" method has an invalid return value).

   modify(fileobj, events, data=None)

      Change a registered file object’s monitored events or attached
      data.

      This is equivalent to "BaseSelector.unregister(fileobj)()"
      followed by "BaseSelector.register(fileobj, events, data)()",
      except that it can be implemented more efficiently.

      This returns a new "SelectorKey" instance, or raises a
      "ValueError" in case of invalid event mask or file descriptor,
      or "KeyError" if the file object is not registered.

   abstractmethod 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 "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.

      Changed in version 3.5: The selector is now retried with a
      recomputed timeout when interrupted by a signal if the signal
      handler did not raise an exception (see **PEP 475** for the
      rationale), instead of returning an empty list of events before
      the timeout.

   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.

   get_key(fileobj)

      Return the key associated with a registered file object.

      This returns the "SelectorKey" instance associated to this file
      object, or raises "KeyError" if the file object is not
      registered.

   abstractmethod get_map()

      Return a mapping of file objects to selector keys.

      This returns a "Mapping" instance mapping registered file
      objects to their associated "SelectorKey" instance.

class selectors.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 selectors.SelectSelector

   "select.select()"-based selector.

class selectors.PollSelector

   "select.poll()"-based selector.

class selectors.EpollSelector

   "select.epoll()"-based selector.

   fileno()

      This returns the file descriptor used by the underlying
      "select.epoll()" object.

class selectors.DevpollSelector

   "select.devpoll()"-based selector.

   fileno()

      This returns the file descriptor used by the underlying
      "select.devpoll()" object.

   New in version 3.5.

class selectors.KqueueSelector

   "select.kqueue()"-based selector.

   fileno()

      This returns the file descriptor used by the underlying
      "select.kqueue()" object.


19.4.3. 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)
