1"""A generally useful event scheduler class.
2
3Each instance of this class manages its own queue.
4No multi-threading is implied; you are supposed to hack that
5yourself, or use a single instance per application.
6
7Each instance is parametrized with two functions, one that is
8supposed to return the current time, one that is supposed to
9implement a delay.  You can implement real-time scheduling by
10substituting time and sleep from built-in module time, or you can
11implement simulated time by writing your own functions.  This can
12also be used to integrate scheduling with STDWIN events; the delay
13function is allowed to modify the queue.  Time can be expressed as
14integers or floating point numbers, as long as it is consistent.
15
16Events are specified by tuples (time, priority, action, argument).
17As in UNIX, lower priority numbers mean higher priority; in this
18way the queue can be maintained as a priority queue.  Execution of the
19event means calling the action function, passing it the argument
20sequence in "argument" (remember that in Python, multiple function
21arguments are be packed in a sequence).
22The action function may be an instance method so it
23has another way to reference private data (besides global variables).
24"""
25
26# XXX The timefunc and delayfunc should have been defined as methods
27# XXX so you can define new kinds of schedulers using subclassing
28# XXX instead of having to define a module or class just to hold
29# XXX the global state of your particular time and delay functions.
30
31import heapq
32from collections import namedtuple
33
34__all__ = ["scheduler"]
35
36Event = namedtuple('Event', 'time, priority, action, argument')
37
38class scheduler:
39    def __init__(self, timefunc, delayfunc):
40        """Initialize a new instance, passing the time and delay
41        functions"""
42        self._queue = []
43        self.timefunc = timefunc
44        self.delayfunc = delayfunc
45
46    def enterabs(self, time, priority, action, argument):
47        """Enter a new event in the queue at an absolute time.
48
49        Returns an ID for the event which can be used to remove it,
50        if necessary.
51
52        """
53        event = Event(time, priority, action, argument)
54        heapq.heappush(self._queue, event)
55        return event # The ID
56
57    def enter(self, delay, priority, action, argument):
58        """A variant that specifies the time as a relative time.
59
60        This is actually the more commonly used interface.
61
62        """
63        time = self.timefunc() + delay
64        return self.enterabs(time, priority, action, argument)
65
66    def cancel(self, event):
67        """Remove an event from the queue.
68
69        This must be presented the ID as returned by enter().
70        If the event is not in the queue, this raises ValueError.
71
72        """
73        self._queue.remove(event)
74        heapq.heapify(self._queue)
75
76    def empty(self):
77        """Check whether the queue is empty."""
78        return not self._queue
79
80    def run(self):
81        """Execute events until the queue is empty.
82
83        When there is a positive delay until the first event, the
84        delay function is called and the event is left in the queue;
85        otherwise, the event is removed from the queue and executed
86        (its action function is called, passing it the argument).  If
87        the delay function returns prematurely, it is simply
88        restarted.
89
90        It is legal for both the delay function and the action
91        function to to modify the queue or to raise an exception;
92        exceptions are not caught but the scheduler's state remains
93        well-defined so run() may be called again.
94
95        A questionable hack is added to allow other threads to run:
96        just after an event is executed, a delay of 0 is executed, to
97        avoid monopolizing the CPU when other threads are also
98        runnable.
99
100        """
101        # localize variable access to minimize overhead
102        # and to improve thread safety
103        q = self._queue
104        delayfunc = self.delayfunc
105        timefunc = self.timefunc
106        pop = heapq.heappop
107        while q:
108            time, priority, action, argument = checked_event = q[0]
109            now = timefunc()
110            if now < time:
111                delayfunc(time - now)
112            else:
113                event = pop(q)
114                # Verify that the event was not removed or altered
115                # by another thread after we last looked at q[0].
116                if event is checked_event:
117                    action(*argument)
118                    delayfunc(0)   # Let other threads run
119                else:
120                    heapq.heappush(q, event)
121
122    @property
123    def queue(self):
124        """An ordered list of upcoming events.
125
126        Events are named tuples with fields for:
127            time, priority, action, arguments
128
129        """
130        # Use heapq to sort the queue rather than using 'sorted(self._queue)'.
131        # With heapq, two events scheduled at the same time will show in
132        # the actual order they would be retrieved.
133        events = self._queue[:]
134        return map(heapq.heappop, [events]*len(events))
135