
``sched`` --- Event scheduler
*****************************

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

class 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.

Example:

   >>> 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.

Example:

   >>> 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


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*.

   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 then 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.
