Doc/library/sched.rst

:mod:`sched` --- Event scheduler
================================

.. module:: sched
   :synopsis: General purpose event scheduler.
.. sectionauthor:: Moshe Zadka <moshez@zadka.site.co.il>

.. index:: single: event scheduling

**Source code:** :source:`Lib/sched.py`

--------------

The :mod:`sched` module defines a class which implements a general purpose event
scheduler:

.. class:: scheduler(timefunc, delayfunc)

   The :class:`scheduler` class defines a generic interface to scheduling events.
   It needs two functions to actually deal with the "outside world" --- *timefunc*
   should be callable without arguments, and return  a number (the "time", in any
   units whatsoever).  The *delayfunc* function should be callable with one
   argument, compatible with the output of *timefunc*, and should delay that many
   time units. *delayfunc* will also be called with the argument ``0`` after each
   event is run to allow other threads an opportunity to run in multi-threaded
   applications.

Example::

   >>> import sched, time
   >>> s = sched.scheduler(time.time, time.sleep)
   >>> def print_time(): print("From print_time", time.time())
   ...
   >>> def print_some_times():
   ...     print(time.time())
   ...     s.enter(5, 1, print_time, ())
   ...     s.enter(10, 1, print_time, ())
   ...     s.run()
   ...     print(time.time())
   ...
   >>> print_some_times()
   930343690.257
   From print_time 930343695.274
   From print_time 930343700.273
   930343700.276

In multi-threaded environments, the :class:`scheduler` class has limitations
with respect to thread-safety, inability to insert a new task before
the one currently pending in a running scheduler, and holding up the main
thread until the event queue is empty.  Instead, the preferred approach
is to use the :class:`threading.Timer` class instead.

Example::

    >>> import time
    >>> from threading import Timer
    >>> def print_time():
    ...     print("From print_time", time.time())
    ...
    >>> def print_some_times():
    ...     print(time.time())
    ...     Timer(5, print_time, ()).start()
    ...     Timer(10, print_time, ()).start()
    ...     time.sleep(11)  # sleep while time-delay events execute
    ...     print(time.time())
    ...
    >>> print_some_times()
    930343690.257
    From print_time 930343695.274
    From print_time 930343700.273
    930343701.301


.. _scheduler-objects:

Scheduler Objects
-----------------

:class:`scheduler` instances have the following methods and attributes:


.. method:: scheduler.enterabs(time, priority, action, argument)

   Schedule a new event. The *time* argument should be a numeric type compatible
   with the return value of the *timefunc* function passed  to the constructor.
   Events scheduled for the same *time* will be executed in the order of their
   *priority*.

   Executing the event means executing ``action(*argument)``.  *argument* must be a
   sequence holding the parameters for *action*.

   Return value is an event which may be used for later cancellation of the event
   (see :meth:`cancel`).


.. method:: scheduler.enter(delay, priority, action, argument)

   Schedule an event for *delay* more time units. Other than the relative time, the
   other arguments, the effect and the return value are the same as those for
   :meth:`enterabs`.


.. method:: scheduler.cancel(event)

   Remove the event from the queue. If *event* is not an event currently in the
   queue, this method will raise a :exc:`ValueError`.


.. method:: scheduler.empty()

   Return true if the event queue is empty.


.. method:: scheduler.run()

   Run all scheduled events. This function will wait  (using the :func:`delayfunc`
   function passed to the constructor) for the next event, then execute it and so
   on until there are no more scheduled events.

   Either *action* or *delayfunc* can raise an exception.  In either case, the
   scheduler will maintain a consistent state and propagate the exception.  If an
   exception is raised by *action*, the event will not be attempted in future calls
   to :meth:`run`.

   If a sequence of events takes longer to run than the time available before the
   next event, the scheduler will simply fall behind.  No events will be dropped;
   the calling code is responsible for canceling  events which are no longer
   pertinent.

.. attribute:: scheduler.queue

   Read-only attribute returning a list of upcoming events in the order they
   will be run.  Each event is shown as a :term:`named tuple` with the
   following fields:  time, priority, action, argument.