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# Module sched -- a generally useful event scheduler class
# Each instance of this class manages its own queue.
# No multi-threading is implied; you are supposed to hack that
# yourself, or use a single instance per application.
#
# Each instance is parametrized with two functions, one that is
# supposed to return the current time, one that is supposed to
# implement a delay. You can implement fine- or course-grained
# real-time scheduling by substituting time and sleep or millitimer
# and millisleep from the built-in module time, or you can implement
# simulated time by writing your own functions. This can also be
# used to integrate scheduling with STDWIN events; the delay function
# is allowed to modify the queue. Time can be expressed as
# integers or floating point numbers, as long as it is consistent.
# Events are specified by tuples (time, priority, action, argument).
# As in UNIX, lower priority numbers mean higher priority; in this
# way the queue can be maintained fully sorted. Execution of the
# event means calling the action function, passing it the argument.
# Remember that in Python, multiple function arguments can be packed
# in a tuple. The action function may be an instance method so it
# has another way to reference private data (besides global variables).
# Parameterless functions or methods cannot be used, however.
# XXX The timefunc and delayfunc should have been defined as methods
# XXX so you can define new kinds of schedulers using subclassing
# XXX instead of having to define a module or class just to hold
# XXX the global state of your particular time and delay functtions.
class scheduler:
#
# Initialize a new instance, passing the time and delay functions
#
def init(self, (timefunc, delayfunc)):
self.queue = []
self.timefunc = timefunc
self.delayfunc = delayfunc
return self
#
# Enter a new event in the queue at an absolute time.
# Returns an ID for the event which can be used
# to remove it, if necessary.
#
def enterabs(self, event):
time, priority, action, argument = event
q = self.queue
# XXX Could use bisection or linear interpolation?
for i in range(len(q)):
qtime, qpri, qact, qarg = q[i]
if time < qtime: break
if time == qtime and priority < qpri: break
else:
i = len(q)
q.insert(i, event)
return event # The ID
#
# A variant that specifies the time as a relative time.
# This is actually the more commonly used interface.
#
def enter(self, (delay, priority, action, argument)):
time = self.timefunc() + delay
return self.enterabs(time, priority, action, argument)
#
# Remove an event from the queue.
# This must be presented the ID as returned by enter().
# If the event is not in the queue, this raises RuntimeError.
#
def cancel(self, event):
self.queue.remove(event)
#
# Check whether the queue is empty.
#
def empty(self):
return len(self.queue) == 0
#
# Run: execute events until the queue is empty.
#
# When there is a positive delay until the first event, the
# delay function is called and the event is left in the queue;
# otherwise, the event is removed from the queue and executed
# (its action function is called, passing it the argument).
# If the delay function returns prematurely, it is simply
# restarted.
#
# It is legal for both the delay function and the action
# function to to modify the queue or to raise an exception;
# exceptions are not caught but the scheduler's state
# remains well-defined so run() may be called again.
#
# A questionably hack is added to allow other threads to run:
# just after an event is executed, a delay of 0 is executed,
# to avoid monopolizing the CPU when other threads are also
# runnable.
#
def run(self):
q = self.queue
while q:
time, priority, action, argument = q[0]
now = self.timefunc()
if now < time:
self.delayfunc(time - now)
else:
del q[0]
void = action(argument)
self.delayfunc(0) # Let other threads run
#
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