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108 lines
3.9 KiB
Python
108 lines
3.9 KiB
Python
# Module sched -- a generally useful event scheduler class
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# Each instance of this class manages its own queue.
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# No multi-threading is implied; you are supposed to hack that
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# yourself, or use a single instance per application.
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#
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# Each instance is parametrized with two functions, one that is
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# supposed to return the current time, one that is supposed to
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# implement a delay. You can implement fine- or course-grained
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# real-time scheduling by substituting time and sleep or millitimer
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# and millisleep from the built-in module time, or you can implement
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# simulated time by writing your own functions. This can also be
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# used to integrate scheduling with STDWIN events; the delay function
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# is allowed to modify the queue. Time can be expressed as
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# integers or floating point numbers, as long as it is consistent.
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# Events are specified by tuples (time, priority, action, argument).
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# As in UNIX, lower priority numbers mean higher priority; in this
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# way the queue can be maintained fully sorted. Execution of the
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# event means calling the action function, passing it the argument.
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# Remember that in Python, multiple function arguments can be packed
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# in a tuple. The action function may be an instance method so it
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# has another way to reference private data (besides global variables).
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# Parameterless functions or methods cannot be used, however.
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# XXX The timefunc and delayfunc should have been defined as methods
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# XXX so you can define new kinds of schedulers using subclassing
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# XXX instead of having to define a module or class just to hold
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# XXX the global state of your particular time and delay functtions.
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class scheduler:
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#
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# Initialize a new instance, passing the time and delay functions
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#
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def init(self, (timefunc, delayfunc)):
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self.queue = []
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self.timefunc = timefunc
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self.delayfunc = delayfunc
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return self
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#
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# Enter a new event in the queue at an absolute time.
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# Returns an ID for the event which can be used
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# to remove it, if necessary.
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#
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def enterabs(self, event):
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time, priority, action, argument = event
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q = self.queue
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# XXX Could use bisection or linear interpolation?
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for i in range(len(q)):
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qtime, qpri, qact, qarg = q[i]
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if time < qtime: break
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if time == qtime and priority < qpri: break
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else:
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i = len(q)
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q.insert(i, event)
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return event # The ID
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#
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# A variant that specifies the time as a relative time.
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# This is actually the more commonly used interface.
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#
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def enter(self, (delay, priority, action, argument)):
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time = self.timefunc() + delay
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return self.enterabs(time, priority, action, argument)
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#
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# Remove an event from the queue.
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# This must be presented the ID as returned by enter().
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# If the event is not in the queue, this raises RuntimeError.
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#
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def cancel(self, event):
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self.queue.remove(event)
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#
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# Check whether the queue is empty.
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#
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def empty(self):
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return len(self.queue) == 0
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#
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# Run: execute events until the queue is empty.
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#
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# When there is a positive delay until the first event, the
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# delay function is called and the event is left in the queue;
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# otherwise, the event is removed from the queue and executed
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# (its action function is called, passing it the argument).
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# If the delay function returns prematurely, it is simply
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# restarted.
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#
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# It is legal for both the delay function and the action
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# function to to modify the queue or to raise an exception;
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# exceptions are not caught but the scheduler's state
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# remains well-defined so run() may be called again.
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#
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# A questionably hack is added to allow other threads to run:
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# just after an event is executed, a delay of 0 is executed,
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# to avoid monopolizing the CPU when other threads are also
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# runnable.
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#
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def run(self):
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q = self.queue
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while q:
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time, priority, action, argument = q[0]
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now = self.timefunc()
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if now < time:
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self.delayfunc(time - now)
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else:
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del q[0]
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void = action(argument)
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self.delayfunc(0) # Let other threads run
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#
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