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# 8.7. `sched` — Event scheduler

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.

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

## 8.7.1. 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.

  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 `RuntimeError`.

`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.

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