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Scheduling tasks for the future
===============================





Let's say we want to run a task X seconds in the future.
The way to do that is defined in the reactor interface :api:`twisted.internet.interfaces.IReactorTime <twisted.internet.interfaces.IReactorTime>` :




.. code-block:: python

    
    from twisted.internet import reactor
    
    def f(s):
        print "this will run 3.5 seconds after it was scheduled: %s" % s
    
    reactor.callLater(3.5, f, "hello, world")
    
    # f() will only be called if the event loop is started.
    reactor.run()



    
If the result of the function is important or if it may be necessary
to handle exceptions it raises, then the :api:`twisted.internet.task.deferLater <twisted.internet.task.deferLater>` utility conveniently
takes care of creating a :api:`twisted.internet.defer.Deferred <Deferred>` and setting up a delayed
call:




.. code-block:: python

    
    from twisted.internet import task
    from twisted.internet import reactor
    
    def f(s):
        return "This will run 3.5 seconds after it was scheduled: %s" % s
    
    d = task.deferLater(reactor, 3.5, f, "hello, world")
    def called(result):
        print result
    d.addCallback(called)
    
    # f() will only be called if the event loop is started.
    reactor.run()



    
If we want a task to run every X seconds repeatedly, we can
use :api:`twisted.internet.task.LoopingCall <twisted.internet.task.LoopingCall>` :




.. code-block:: python

    
    from twisted.internet import task
    from twisted.internet import reactor
    
    def runEverySecond():
        print "a second has passed"
    
    l = task.LoopingCall(runEverySecond)
    l.start(1.0) # call every second
    
    # l.stop() will stop the looping calls
    reactor.run()



    
If we want to cancel a task that we've scheduled:




.. code-block:: python

    
    from twisted.internet import reactor
    
    def f():
        print "I'll never run."
    
    callID = reactor.callLater(5, f)
    callID.cancel()
    reactor.run()

     

    
As with all reactor-based code, in order for scheduling to work the reactor must be started using ``reactor.run()`` .