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+"""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).
+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).
+The action function may be an instance method so it
+has another way to reference private data (besides global variables).
+"""
+
+# XXX The timefunc and delayfunc should have been defined as methods
+# XXX so you can define new kinds of schedulers using subclassing
+# XXX instead of having to define a module or class just to hold
+# XXX the global state of your particular time and delay functions.
+
+import heapq
+from collections import namedtuple
+
+__all__ = ["scheduler"]
+
+Event = namedtuple('Event', 'time, priority, action, argument')
+
+class scheduler:
+    def __init__(self, timefunc, delayfunc):
+        """Initialize a new instance, passing the time and delay
+        functions"""
+        self._queue = []
+        self.timefunc = timefunc
+        self.delayfunc = delayfunc
+
+    def enterabs(self, time, priority, action, argument):
+        """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.
+
+        """
+        event = Event(time, priority, action, argument)
+        heapq.heappush(self._queue, event)
+        return event # The ID
+
+    def enter(self, delay, priority, action, argument):
+        """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)
+
+    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.
+
+        """
+        self._queue.remove(event)
+        heapq.heapify(self._queue)
+
+    def empty(self):
+        """Check whether the queue is empty."""
+        return not self._queue
+
+    def run(self):
+        """Execute events until the queue is empty.
+
+        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
+        q = self._queue
+        delayfunc = self.delayfunc
+        timefunc = self.timefunc
+        pop = heapq.heappop
+        while q:
+            time, priority, action, argument = checked_event = q[0]
+            now = timefunc()
+            if now < time:
+                delayfunc(time - now)
+            else:
+                event = pop(q)
+                # Verify that the event was not removed or altered
+                # by another thread after we last looked at q[0].
+                if event is checked_event:
+                    action(*argument)
+                    delayfunc(0)   # Let other threads run
+                else:
+                    heapq.heappush(q, event)
+
+    @property
+    def queue(self):
+        """An ordered list of upcoming events.
+
+        Events are named tuples with fields for:
+            time, priority, action, arguments
+
+        """
+        # 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.
+        events = self._queue[:]
+        return map(heapq.heappop, [events]*len(events))