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))