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6.9.1 Scheduler Objects

`scheduler` instances have the following methods:

`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 `apply(action, argument)`. argument must be a tuple holding the parameters for action.

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

`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()`.

`cancel` (event)
  Remove the event from the queue. If event is not an event currently in the queue, this method will raise a `RuntimeError`.

`empty` ()
  Return true if the event queue is empty.

`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 cancelling events which are no longer pertinent.