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.

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

8.8.1. 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.

New in version 2.6.