"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)

   "scheduler" 类定义了一个调度事件的通用接口。 它需要两个函数来实际处
   理“外部世界” —— *timefunc* 应当不带参数地调用，并返回一个数字（“时
   间”，可以为任意单位）。 *delayfunc* 函数应当带一个参数调用，与
   *timefunc* 的输出相兼容，并且应当延迟其所指定的时间单位。 每个事件
   运行后还将调用 *delayfunc* 并传入参数 "0" 以允许其他线程有机会在多
   线程应用中运行。

   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


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 版更變: *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 版更變: *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()

   如果事件队列为空则返回 "True"。

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 版更變: *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.
