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:mod:`sched` --- Event scheduler

.. module:: sched
   :synopsis: General purpose event scheduler.


.. sectionauthor:: Moshe Zadka <moshez@zadka.site.co.il>

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

.. index:: single: event scheduling

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

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). If time.monotonic is not available, the timefunc default is time.time instead. 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.

.. versionchanged:: 3.3
   *timefunc* and *delayfunc* parameters are optional.


.. versionchanged:: 3.3
   :class:`scheduler` class can be safely used in multi-threaded
   environments.

Example:

>>> import sched, time
>>> s = sched.scheduler(time.time, time.sleep)
>>> def print_time(a='default'):
...     print("From print_time", time.time(), a)
...
>>> def print_some_times():
...     print(time.time())
...     s.enter(10, 1, print_time)
...     s.enter(5, 2, print_time, argument=('positional',))
...     s.enter(5, 1, print_time, kwargs={'a': 'keyword'})
...     s.run()
...     print(time.time())
...
>>> print_some_times()
930343690.257
From print_time 930343695.274 positional
From print_time 930343695.275 keyword
From print_time 930343700.273 default
930343700.276

Scheduler Objects

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

.. method:: scheduler.enterabs(time, priority, action, argument=(), kwargs={})

   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, **kwargs)``.
   *argument* is a sequence holding the positional arguments for *action*.
   *kwargs* is a dictionary holding the keyword arguments for *action*.

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

   .. versionchanged:: 3.3
      *argument* parameter is optional.

   .. versionadded:: 3.3
      *kwargs* parameter was added.



.. method:: scheduler.enter(delay, priority, action, argument=(), kwargs={})

   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`.

   .. versionchanged:: 3.3
      *argument* parameter is optional.

   .. versionadded:: 3.3
      *kwargs* parameter was added.


.. 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(blocking=True)

   Run all scheduled events. This method 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.

   If *blocking* is false executes the scheduled events due to expire soonest
   (if any) and then return the deadline of the next scheduled call in the
   scheduler (if any).

   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.

   .. versionadded:: 3.3
      *blocking* parameter was added.


.. 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, kwargs.