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sched.py
"""A generally useful event scheduler class. Each instance of this class manages its own queue. No multi-threading is implied; you are supposed to hack that yourself, or use a single instance per application. Each instance is parametrized with two functions, one that is supposed to return the current time, one that is supposed to implement a delay. You can implement real-time scheduling by substituting time and sleep from built-in module time, or you can implement simulated time by writing your own functions. This can also be used to integrate scheduling with STDWIN events; the delay function is allowed to modify the queue. Time can be expressed as integers or floating point numbers, as long as it is consistent. Events are specified by tuples (time, priority, action, argument, kwargs). As in UNIX, lower priority numbers mean higher priority; in this way the queue can be maintained as a priority queue. Execution of the event means calling the action function, passing it the argument sequence in "argument" (remember that in Python, multiple function arguments are be packed in a sequence) and keyword parameters in "kwargs". The action function may be an instance method so it has another way to reference private data (besides global variables). """ import time import heapq from collections import namedtuple import threading from time import monotonic as _time __all__ = ["scheduler"] class Event(namedtuple('Event', 'time, priority, action, argument, kwargs')): __slots__ = [] def __eq__(s, o): return (s.time, s.priority) == (o.time, o.priority) def __lt__(s, o): return (s.time, s.priority) < (o.time, o.priority) def __le__(s, o): return (s.time, s.priority) <= (o.time, o.priority) def __gt__(s, o): return (s.time, s.priority) > (o.time, o.priority) def __ge__(s, o): return (s.time, s.priority) >= (o.time, o.priority) Event.time.__doc__ = ('''Numeric type compatible with the return value of the timefunc function passed to the constructor.''') Event.priority.__doc__ = ('''Events scheduled for the same time will be executed in the order of their priority.''') Event.action.__doc__ = ('''Executing the event means executing action(*argument, **kwargs)''') Event.argument.__doc__ = ('''argument is a sequence holding the positional arguments for the action.''') Event.kwargs.__doc__ = ('''kwargs is a dictionary holding the keyword arguments for the action.''') _sentinel = object() class scheduler: def __init__(self, timefunc=_time, delayfunc=time.sleep): """Initialize a new instance, passing the time and delay functions""" self._queue = [] self._lock = threading.RLock() self.timefunc = timefunc self.delayfunc = delayfunc def enterabs(self, time, priority, action, argument=(), kwargs=_sentinel): """Enter a new event in the queue at an absolute time. Returns an ID for the event which can be used to remove it, if necessary. """ if kwargs is _sentinel: kwargs = {} event = Event(time, priority, action, argument, kwargs) with self._lock: heapq.heappush(self._queue, event) return event # The ID def enter(self, delay, priority, action, argument=(), kwargs=_sentinel): """A variant that specifies the time as a relative time. This is actually the more commonly used interface. """ time = self.timefunc() + delay return self.enterabs(time, priority, action, argument, kwargs) def cancel(self, event): """Remove an event from the queue. This must be presented the ID as returned by enter(). If the event is not in the queue, this raises ValueError. """ with self._lock: self._queue.remove(event) heapq.heapify(self._queue) def empty(self): """Check whether the queue is empty.""" with self._lock: return not self._queue def run(self, blocking=True): """Execute events until the queue is empty. If blocking is False executes the scheduled events due to expire soonest (if any) and then return the deadline of the next scheduled call in the scheduler. When there is a positive delay until the first event, the delay function is called and the event is left in the queue; otherwise, the event is removed from the queue and executed (its action function is called, passing it the argument). If the delay function returns prematurely, it is simply restarted. It is legal for both the delay function and the action function to modify the queue or to raise an exception; exceptions are not caught but the scheduler's state remains well-defined so run() may be called again. A questionable hack is added to allow other threads to run: just after an event is executed, a delay of 0 is executed, to avoid monopolizing the CPU when other threads are also runnable. """ # localize variable access to minimize overhead # and to improve thread safety lock = self._lock q = self._queue delayfunc = self.delayfunc timefunc = self.timefunc pop = heapq.heappop while True: with lock: if not q: break time, priority, action, argument, kwargs = q[0] now = timefunc() if time > now: delay = True else: delay = False pop(q) if delay: if not blocking: return time - now delayfunc(time - now) else: action(*argument, **kwargs) delayfunc(0) # Let other threads run @property def queue(self): """An ordered list of upcoming events. Events are named tuples with fields for: time, priority, action, arguments, kwargs """ # Use heapq to sort the queue rather than using 'sorted(self._queue)'. # With heapq, two events scheduled at the same time will show in # the actual order they would be retrieved. with self._lock: events = self._queue[:] return list(map(heapq.heappop, [events]*len(events)))
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