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 Remember that in Python, multiple function arguments can be packed
  21 in a tuple.   The action function may be an instance method so it
  22 has another way to reference private data (besides global variables).
  23 Parameterless functions or methods cannot be used, however.
  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
  33 __all__ = ["scheduler"]
  34
  35 class scheduler:
  36     def __init__(self, timefunc, delayfunc):
  37         """Initialize a new instance, passing the time and delay
  38         functions"""
  39         self.queue = []
  40         self.timefunc = timefunc
  41         self.delayfunc = delayfunc
  42
  43     def enterabs(self, time, priority, action, argument):
  44         """Enter a new event in the queue at an absolute time.
  45
  46         Returns an ID for the event which can be used to remove it,
  47         if necessary.
  48
  49         """
  50         event = time, priority, action, argument
  51         heapq.heappush(self.queue, event)
  52         return event # The ID
  53
  54     def enter(self, delay, priority, action, argument):
  55         """A variant that specifies the time as a relative time.
  56
  57         This is actually the more commonly used interface.
  58
  59         """
  60         time = self.timefunc() + delay
  61         return self.enterabs(time, priority, action, argument)
  62
  63     def cancel(self, event):
  64         """Remove an event from the queue.
  65
  66         This must be presented the ID as returned by enter().
  67         If the event is not in the queue, this raises RuntimeError.
  68
  69         """
  70         self.queue.remove(event)
  71         heapq.heapify(self.queue)
  72
  73     def empty(self):
  74         """Check whether the queue is empty."""
  75         return not not self.queue
  76
  77     def run(self):
  78         """Execute events until the queue is empty.
  79
  80         When there is a positive delay until the first event, the
  81         delay function is called and the event is left in the queue;
  82         otherwise, the event is removed from the queue and executed
  83         (its action function is called, passing it the argument).  If
  84         the delay function returns prematurely, it is simply
  85         restarted.
  86
  87         It is legal for both the delay function and the action
  88         function to to modify the queue or to raise an exception;
  89         exceptions are not caught but the scheduler's state remains
  90         well-defined so run() may be called again.
  91
  92         A questionably hack is added to allow other threads to run:
  93         just after an event is executed, a delay of 0 is executed, to
  94         avoid monopolizing the CPU when other threads are also
  95         runnable.
  96
  97         """
  98         # localize variable access to minimize overhead
  99         # and to improve thread safety
 100         q = self.queue
 101         delayfunc = self.delayfunc
 102         timefunc = self.timefunc
 103         pop = heapq.heappop
 104         while q:
 105             time, priority, action, argument = checked_event = q[0]
 106             now = timefunc()
 107             if now < time:
 108                 delayfunc(time - now)
 109             else:
 110                 event = pop(q)
 111                 # Verify that the event was not removed or altered
 112                 # by another thread after we last looked at q[0].
 113                 if event is checked_event:
 114                     void = action(*argument)
 115                     delayfunc(0)   # Let other threads run
 116                 else:
 117                     heapq.heappush(event)