"sched" --- Event scheduler
***************************

**原始碼：**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).  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.

   在 3.3 版的變更: *timefunc* and *delayfunc* parameters are
   optional.

   在 3.3 版的變更: "scheduler" class can be safely used in multi-
   threaded environments.

範例：

   >>> 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" 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()").

   在 3.3 版的變更: *argument* parameter is optional.

   在 3.3 版的變更: 新增 *kwargs* 參數。

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()".

   在 3.3 版的變更: *argument* parameter is optional.

   在 3.3 版的變更: 新增 *kwargs* 參數。

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.

   在 3.3 版的變更: 新增 *blocking* 參數。

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.
