:mod:`select` -- wait for events on a set of streams
====================================================

.. module:: select
   :synopsis: wait for events on a set of streams

|see_cpython_module| :mod:`python:select`.

This module provides functions to efficiently wait for events on multiple
`streams <stream>` (select streams which are ready for operations).

Functions
---------

.. function:: poll()

   Create an instance of the Poll class.

.. function:: select(rlist, wlist, xlist[, timeout])

   Wait for activity on a set of objects.

   This function is provided by some MicroPython ports for compatibility
   and is not efficient. Usage of :class:`Poll` is recommended instead.

.. _class: Poll

class ``Poll``
--------------

Methods
~~~~~~~

.. method:: poll.register(obj[, eventmask])

   Register `stream` *obj* for polling. *eventmask* is logical OR of:

   * ``select.POLLIN``  - data available for reading
   * ``select.POLLOUT`` - more data can be written

   Note that flags like ``select.POLLHUP`` and ``select.POLLERR`` are
   *not* valid as input eventmask (these are unsolicited events which
   will be returned from `poll()` regardless of whether they are asked
   for). This semantics is per POSIX.

   *eventmask* defaults to ``select.POLLIN | select.POLLOUT``.

   It is OK to call this function multiple times for the same *obj*.
   Successive calls will update *obj*'s eventmask to the value of
   *eventmask* (i.e. will behave as `modify()`).

.. method:: poll.unregister(obj)

   Unregister *obj* from polling.

.. method:: poll.modify(obj, eventmask)

   Modify the *eventmask* for *obj*. If *obj* is not registered, `OSError`
   is raised with error of ENOENT.

.. method:: poll.poll(timeout=-1, /)

   Wait for at least one of the registered objects to become ready or have an
   exceptional condition, with optional timeout in milliseconds (if *timeout*
   arg is not specified or -1, there is no timeout).

   Returns list of (``obj``, ``event``, ...) tuples. There may be other elements in
   tuple, depending on a platform and version, so don't assume that its size is 2.
   The ``event`` element specifies which events happened with a stream and
   is a combination of ``select.POLL*`` constants described above. Note that
   flags ``select.POLLHUP`` and ``select.POLLERR`` can be returned at any time
   (even if were not asked for), and must be acted on accordingly (the
   corresponding stream unregistered from poll and likely closed), because
   otherwise all further invocations of `poll()` may return immediately with
   these flags set for this stream again.

   In case of timeout, an empty list is returned.

   Calling ``poll.poll`` is guaranteed to call pending callback functions
   before entering the polling loop.

   .. admonition:: Difference to CPython
      :class: attention

      Tuples returned may contain more than 2 elements as described above.

.. method:: poll.ipoll(timeout=-1, flags=0, /)

   Like :meth:`poll.poll`, but instead returns an iterator which yields a
   `callee-owned tuple`. This function provides an efficient, allocation-free
   way to poll on streams.

   If *flags* is 1, one-shot behaviour for events is employed: streams for
   which events happened will have their event masks automatically reset
   (equivalent to ``poll.modify(obj, 0)``), so new events for such a stream
   won't be processed until new mask is set with `poll.modify()`. This
   behaviour is useful for asynchronous I/O schedulers.

   Calling ``poll.ipoll`` is guaranteed to call pending callback functions
   before entering the polling loop.

   .. admonition:: Difference to CPython
      :class: attention

      This function is a MicroPython extension.
