117 lines
4.4 KiB
Python
117 lines
4.4 KiB
Python
"""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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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 real-time scheduling by
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substituting time and sleep from built-in module time, or you can
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implement simulated time by writing your own functions. This can
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also be used to integrate scheduling with STDWIN events; the delay
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function 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 as a priority queue. 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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"""
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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 functions.
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import heapq
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__all__ = ["scheduler"]
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class scheduler:
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def __init__(self, timefunc, delayfunc):
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"""Initialize a new instance, passing the time and delay
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functions"""
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self.queue = []
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self.timefunc = timefunc
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self.delayfunc = delayfunc
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def enterabs(self, time, priority, action, argument):
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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 to remove it,
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if necessary.
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"""
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event = time, priority, action, argument
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heapq.heappush(self.queue, event)
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return event # The ID
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def enter(self, delay, priority, action, argument):
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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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time = self.timefunc() + delay
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return self.enterabs(time, priority, action, argument)
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def cancel(self, event):
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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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self.queue.remove(event)
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heapq.heapify(self.queue)
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def empty(self):
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"""Check whether the queue is empty."""
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return not self.queue
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def run(self):
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"""Execute events until the queue is empty.
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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). If
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the delay function returns prematurely, it is simply
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restarted.
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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 remains
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well-defined so run() may be called again.
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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, to
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avoid monopolizing the CPU when other threads are also
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runnable.
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"""
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# localize variable access to minimize overhead
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# and to improve thread safety
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q = self.queue
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delayfunc = self.delayfunc
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timefunc = self.timefunc
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pop = heapq.heappop
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while q:
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time, priority, action, argument = checked_event = q[0]
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now = timefunc()
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if now < time:
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delayfunc(time - now)
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else:
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event = pop(q)
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# Verify that the event was not removed or altered
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# by another thread after we last looked at q[0].
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if event is checked_event:
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void = action(*argument)
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delayfunc(0) # Let other threads run
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else:
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heapq.heappush(event)
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