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:mod:`select` --- Waiting for I/O completion
============================================
.. module:: select
:synopsis: Wait for I/O completion on multiple streams.
This module provides access to the :cfunc:`select` and :cfunc:`poll` functions
available in most operating systems. Note that on Windows, it only works for
sockets; on other operating systems, it also works for other file types (in
particular, on Unix, it works on pipes). It cannot be used on regular files to
determine whether a file has grown since it was last read.
The module defines the following:
.. exception:: error
The exception raised when an error occurs. The accompanying value is a pair
containing the numeric error code from :cdata:`errno` and the corresponding
string, as would be printed by the C function :cfunc:`perror`.
.. function:: poll()
(Not supported by all operating systems.) Returns a polling object, which
supports registering and unregistering file descriptors, and then polling them
for I/O events; see section :ref:`poll-objects` below for the methods supported
by polling objects.
.. function:: select(iwtd, owtd, ewtd[, timeout])
This is a straightforward interface to the Unix :cfunc:`select` system call.
The first three arguments are sequences of 'waitable objects': either
integers representing file descriptors or objects with a parameterless method
named :meth:`fileno` returning such an integer. The three sequences of
waitable objects are for input, output and 'exceptional conditions',
respectively. Empty sequences are allowed, but acceptance of three empty
sequences is platform-dependent. (It is known to work on Unix but not on
Windows.) The optional *timeout* argument specifies a time-out as a floating
point number in seconds. When the *timeout* argument is omitted the function
blocks until at least one file descriptor is ready. A time-out value of zero
specifies a poll and never blocks.
The return value is a triple of lists of objects that are ready: subsets of the
first three arguments. When the time-out is reached without a file descriptor
becoming ready, three empty lists are returned.
.. index::
single: socket() (in module socket)
single: popen() (in module os)
Among the acceptable object types in the sequences are Python file objects (e.g.
``sys.stdin``, or objects returned by :func:`open` or :func:`os.popen`), socket
objects returned by :func:`socket.socket`. You may also define a :dfn:`wrapper`
class yourself, as long as it has an appropriate :meth:`fileno` method (that
really returns a file descriptor, not just a random integer).
.. note::
.. index:: single: WinSock
File objects on Windows are not acceptable, but sockets are. On Windows, the
underlying :cfunc:`select` function is provided by the WinSock library, and does
not handle file descriptors that don't originate from WinSock.
.. _poll-objects:
Polling Objects
---------------
The :cfunc:`poll` system call, supported on most Unix systems, provides better
scalability for network servers that service many, many clients at the same
time. :cfunc:`poll` scales better because the system call only requires listing
the file descriptors of interest, while :cfunc:`select` builds a bitmap, turns
on bits for the fds of interest, and then afterward the whole bitmap has to be
linearly scanned again. :cfunc:`select` is O(highest file descriptor), while
:cfunc:`poll` is O(number of file descriptors).
.. method:: poll.register(fd[, eventmask])
Register a file descriptor with the polling object. Future calls to the
:meth:`poll` method will then check whether the file descriptor has any pending
I/O events. *fd* can be either an integer, or an object with a :meth:`fileno`
method that returns an integer. File objects implement :meth:`fileno`, so they
can also be used as the argument.
*eventmask* is an optional bitmask describing the type of events you want to
check for, and can be a combination of the constants :const:`POLLIN`,
:const:`POLLPRI`, and :const:`POLLOUT`, described in the table below. If not
specified, the default value used will check for all 3 types of events.
+-------------------+------------------------------------------+
| Constant | Meaning |
+===================+==========================================+
| :const:`POLLIN` | There is data to read |
+-------------------+------------------------------------------+
| :const:`POLLPRI` | There is urgent data to read |
+-------------------+------------------------------------------+
| :const:`POLLOUT` | Ready for output: writing will not block |
+-------------------+------------------------------------------+
| :const:`POLLERR` | Error condition of some sort |
+-------------------+------------------------------------------+
| :const:`POLLHUP` | Hung up |
+-------------------+------------------------------------------+
| :const:`POLLNVAL` | Invalid request: descriptor not open |
+-------------------+------------------------------------------+
Registering a file descriptor that's already registered is not an error, and has
the same effect as registering the descriptor exactly once.
.. method:: poll.unregister(fd)
Remove a file descriptor being tracked by a polling object. Just like the
:meth:`register` method, *fd* can be an integer or an object with a
:meth:`fileno` method that returns an integer.
Attempting to remove a file descriptor that was never registered causes a
:exc:`KeyError` exception to be raised.
.. method:: poll.poll([timeout])
Polls the set of registered file descriptors, and returns a possibly-empty list
containing ``(fd, event)`` 2-tuples for the descriptors that have events or
errors to report. *fd* is the file descriptor, and *event* is a bitmask with
bits set for the reported events for that descriptor --- :const:`POLLIN` for
waiting input, :const:`POLLOUT` to indicate that the descriptor can be written
to, and so forth. An empty list indicates that the call timed out and no file
descriptors had any events to report. If *timeout* is given, it specifies the
length of time in milliseconds which the system will wait for events before
returning. If *timeout* is omitted, negative, or :const:`None`, the call will
block until there is an event for this poll object.
|
