Lib/sched.py

   1 """A generally useful event scheduler class.
   2
   3 Each instance of this class manages its own queue.
   4 No multi-threading is implied; you are supposed to hack that
   5 yourself, or use a single instance per application.
   6
   7 Each instance is parametrized with two functions, one that is
   8 supposed to return the current time, one that is supposed to
   9 implement a delay.  You can implement real-time scheduling by
  10 substituting time and sleep from built-in module time, or you can
  11 implement simulated time by writing your own functions.  This can
  12 also be used to integrate scheduling with STDWIN events; the delay
  13 function is allowed to modify the queue.  Time can be expressed as
  14 integers or floating point numbers, as long as it is consistent.
  15
  16 Events are specified by tuples (time, priority, action, argument).
  17 As in UNIX, lower priority numbers mean higher priority; in this
  18 way the queue can be maintained as a priority queue.  Execution of the
  19 event means calling the action function, passing it the argument
  20 sequence in "argument" (remember that in Python, multiple function
  21 arguments are be packed in a sequence).
  22 The action function may be an instance method so it
  23 has 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
  31 import heapq
  32 from collections import namedtuple
  33
  34 __all__ = ["scheduler"]
  35
  36 Event = namedtuple('Event', 'time, priority, action, argument')
  37
  38 class 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 RuntimeError.
  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))