17.6. "sched" --- Event scheduler
*********************************

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

======================================================================

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

class sched.scheduler(timefunc=time.monotonic, delayfunc=time.sleep)

   The "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.

   3.3 版更變: *timefunc* and *delayfunc* parameters are optional.

   3.3 版更變: "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


17.6.1. Scheduler Objects
=========================

"scheduler" instances have the following methods and attributes:

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 "cancel()").

   3.3 版更變: *argument* parameter is optional.

   3.3 版新加入: *kwargs* parameter was added.

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 "enterabs()".

   3.3 版更變: *argument* parameter is optional.

   3.3 版新加入: *kwargs* parameter was added.

scheduler.cancel(event)

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

scheduler.empty()

   Return true if the event queue is empty.

scheduler.run(blocking=True)

   Run all scheduled events. This method will wait  (using the
   "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 "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.

   3.3 版新加入: *blocking* parameter was added.

scheduler.queue

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