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 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)