: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:
.. class:: scheduler(timefunc=time.monotonic, delayfunc=time.sleep)
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.
.. 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',))
... # despite having higher priority, 'keyword' runs after 'positional' as enter() is relative
... s.enter(5, 1, print_time, kwargs={'a': 'keyword'})
... s.enterabs(1_650_000_000, 10, print_time, argument=("first enterabs",))
... s.enterabs(1_650_000_000, 5, print_time, argument=("second enterabs",))
... s.run()
... print(time.time())
...
>>> print_some_times()
1652342830.3640375
From print_time 1652342830.3642538 second enterabs
From print_time 1652342830.3643398 first enterabs
From print_time 1652342835.3694863 positional
From print_time 1652342835.3696074 keyword
From print_time 1652342840.369612 default
1652342840.3697174
.. _scheduler-objects:
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*. A lower number represents a higher 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.
.. versionchanged:: 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.
.. versionchanged:: 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.
.. versionchanged:: 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.
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