8.8. "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, delayfunc)

   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.

示例:

   >>> import sched, time
   >>> s = sched.scheduler(time.time, time.sleep)
   >>> def print_time(): print "From print_time", time.time()
   ...
   >>> def print_some_times():
   ...     print time.time()
   ...     s.enter(5, 1, print_time, ())
   ...     s.enter(10, 1, print_time, ())
   ...     s.run()
   ...     print time.time()
   ...
   >>> print_some_times()
   930343690.257
   From print_time 930343695.274
   From print_time 930343700.273
   930343700.276

In multi-threaded environments, the "scheduler" class has limitations
with respect to thread-safety, inability to insert a new task before
the one currently pending in a running scheduler, and holding up the
main thread until the event queue is empty.  Instead, the preferred
approach is to use the "threading.Timer" class instead.

示例:

   >>> import time
   >>> from threading import Timer
   >>> def print_time():
   ...     print "From print_time", time.time()
   ...
   >>> def print_some_times():
   ...     print time.time()
   ...     Timer(5, print_time, ()).start()
   ...     Timer(10, print_time, ()).start()
   ...     time.sleep(11)  # sleep while time-delay events execute
   ...     print time.time()
   ...
   >>> print_some_times()
   930343690.257
   From print_time 930343695.274
   From print_time 930343700.273
   930343701.301


8.8.1. Scheduler Objects
========================

"scheduler" instances have the following methods and attributes:

scheduler.enterabs(time, priority, action, argument)

   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)".
   *argument* must be a sequence holding the parameters for *action*.

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

scheduler.enter(delay, priority, action, argument)

   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()".

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()

   Run all scheduled events. This function 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.

   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.

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.

   2.6 新版功能.
