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sched Module Complexity¶

The sched module runs callbacks at scheduled times from a single priority queue. A scheduler keeps its events in a heapq heap ordered by time, then priority, then insertion order, and it has no thread of its own: run() pops and calls events on the calling thread, and waits between them with the delayfunc it was given.

n is the events in the queue, counting any that actions enter while run() is working through it, and d is the events run(blocking=False) finds due. Event times are numbers, so comparing two events is O(1), and timefunc() is taken as O(1) with each wait lasting as long as it was asked to. The bounds cover the scheduler's own work; the actions it calls, including passing them their arguments, and the time spent in delayfunc are extra.

Complexity Reference¶

scheduler¶

Operation	Time	Space	Notes
`sched.scheduler(timefunc=time.monotonic, delayfunc=time.sleep)`	O(1)	O(1)	An empty queue; pass your own pair to run on simulated time
`scheduler.enterabs(time, priority, action, argument=(), kwargs={})`	O(log n)	O(1)	One heap push; returns the `Event`, which is the handle `cancel()` takes
`scheduler.enter(delay, priority, action, argument=(), kwargs={})`	O(log n)	O(1)	One `timefunc()` call, then `enterabs()` at that time plus `delay`
`scheduler.cancel(event)`	O(n)	O(1)	Rebuilds the whole heap, even for the next event due; raises `ValueError` if the event already ran or was cancelled
`scheduler.empty()`	O(1)	O(1)
`scheduler.run(blocking=True)`	O(n log n)	O(1)	One heap pop per event, and `delayfunc(0)` after every action
`scheduler.run(blocking=False)`	O(1 + d log n)	O(1)	Runs only the due events, then returns the delay until the next one, or `None` if none is left
`scheduler.queue`	O(n log n)	O(n)	A new list sorted in run order on every read
`scheduler.timefunc`, `scheduler.delayfunc`	O(1)	O(1)	The two functions the scheduler was built with

Event¶

Operation	Time	Space	Notes
`Event.time`, `Event.priority`, `Event.sequence`, `Event.action`, `Event.argument`, `Event.kwargs`	O(1)	O(1)	A named tuple; events order by time, then priority, then `sequence`, so events that tie on both run in the order they were entered

Scheduling Events¶

Relative and Absolute Times¶

enter() reads the clock once and adds the delay; enterabs() takes the time as given. Either way the event goes into the heap in O(log n). The examples on this page that run the scheduler drive it with a simulated clock, so they finish instantly: its sleep() advances the time instead of waiting.

import sched

class Clock:
    def __init__(self):
        self.now = 0
    def time(self):
        return self.now
    def sleep(self, seconds):
        self.now += seconds

clock = Clock()
scheduler = sched.scheduler(clock.time, clock.sleep)  # O(1)
ran = []

scheduler.enter(10, 1, ran.append, argument=('ten',))        # O(log n)
scheduler.enterabs(5, 2, ran.append, argument=('low',))      # O(log n)
scheduler.enterabs(5, 1, ran.append, argument=('high',))     # lower number runs first
scheduler.enterabs(5, 1, ran.append, argument=('entered later',))

scheduler.run()  # O(n log n)
assert ran == ['high', 'entered later', 'low', 'ten']
assert clock.now == 10

Running Without Blocking¶

run(blocking=False) pops only the events that are due and returns. What it returns is the delay until the next event, not that event's time, so it can be passed straight to whatever wait your own loop uses.

import sched

class Clock:
    def __init__(self):
        self.now = 0
    def time(self):
        return self.now
    def sleep(self, seconds):
        self.now += seconds

clock = Clock()
scheduler = sched.scheduler(clock.time, clock.sleep)
ran = []
for when in (1, 2, 10):
    scheduler.enterabs(when, 1, ran.append, argument=(when,))

clock.now = 5
assert scheduler.run(blocking=False) == 5  # O(1 + d log n): ran the two due events
assert ran == [1, 2]

clock.now = 10
assert scheduler.run(blocking=False) is None  # nothing left to wait for
assert ran == [1, 2, 10]

Cancelling Events¶

cancel() finds the event with a linear scan and then rebuilds the heap, so it is O(n) wherever the event sits, including at the front. The event is gone once it has run, and cancelling it then raises ValueError.

import sched

class Clock:
    def __init__(self):
        self.now = 0
    def time(self):
        return self.now
    def sleep(self, seconds):
        self.now += seconds

clock = Clock()
scheduler = sched.scheduler(clock.time, clock.sleep)
ran = []

event = scheduler.enter(5, 1, ran.append, argument=('cancelled',))  # O(log n)
scheduler.enter(6, 1, ran.append, argument=('kept',))

scheduler.cancel(event)  # O(n) - scan, then heapify
scheduler.run()
assert ran == ['kept']

try:
    scheduler.cancel(event)
except ValueError:
    pass
else:
    raise AssertionError('an event no longer queued was cancelled')

Inspecting the Queue¶

queue copies the heap and pops every copied event to put them in run order, which is O(n log n) time and O(n) memory on each read. To ask whether anything is left, empty() is O(1).

import sched

scheduler = sched.scheduler()
first = scheduler.enterabs(2, 1, print)
second = scheduler.enterabs(1, 1, print)

upcoming = scheduler.queue  # O(n log n) - a new sorted list
assert upcoming == [second, first]
assert upcoming[0].time == 1 and upcoming[0].action is print
assert scheduler.queue is not upcoming  # rebuilt on every read

assert not scheduler.empty()  # O(1)

Common Patterns¶

Periodic Tasks¶

An action can enter its own next run. Each repetition is one O(log n) push and one O(log n) pop.

import sched

class Clock:
    def __init__(self):
        self.now = 0
    def time(self):
        return self.now
    def sleep(self, seconds):
        self.now += seconds

clock = Clock()
scheduler = sched.scheduler(clock.time, clock.sleep)
ticks = []

def tick(remaining):
    ticks.append(clock.time())
    if remaining > 1:
        scheduler.enter(10, 1, tick, argument=(remaining - 1,))  # O(log n)

scheduler.enter(0, 1, tick, argument=(3,))
scheduler.run()
assert ticks == [0, 10, 20]

Cancelling Many Events¶

Cancelling k events one at a time costs O(k·n). Where most scheduled events end up cancelled, a flag the action checks makes each cancellation O(1); the event stays queued and costs its O(log n) pop when its time comes.

import sched

class Clock:
    def __init__(self):
        self.now = 0
    def time(self):
        return self.now
    def sleep(self, seconds):
        self.now += seconds

clock = Clock()
scheduler = sched.scheduler(clock.time, clock.sleep)
cancelled = set()
ran = []

def action(name):
    if name not in cancelled:
        ran.append(name)

for index in range(100):
    scheduler.enter(index, 1, action, argument=(index,))  # O(log n)

cancelled.update(range(0, 100, 2))  # O(1) per event, no heap rebuild
scheduler.run()
assert ran == list(range(1, 100, 2))

Performance Best Practices¶

✅ Do:

Use empty() to check for pending work; it is O(1) where queue is O(n log n)
Keep the Event that enter() returns if you may cancel it, rather than finding it again in queue

❌ Avoid:

Reading queue in a loop - every read builds and sorts a new list
Cancelling most of a large queue one event at a time - each cancel() is O(n)

Version Notes¶

All Python 3: run(blocking=False) returns the delay until the next event, not its scheduled time

heapq - the heap behind the queue, and the O(log n) push and pop it pays
asyncio - loop.call_later() and loop.call_at() schedule callbacks inside an event loop
threading - threading.Timer runs one delayed call on its own thread