[Python Std Library] Data Types : sched — Event scheduler

翻譯自 這裡
Preface :
sched 模組提供類別作為一般用途的時程事件功能. 首先來看看建構子 :
- class sched.scheduler(timefunc, delayfunc)

The scheduler class defines a generic interface to scheduling events. It needs two functions to actually deal with the “outside world” — timefunc should be callable without arguments, and return a number (the “time”, in any units whatsoever). The delayfunc function should be callable with one argument, compatible with the output of timefunc, and should delay that many time units. delayfunc will also be called with the argument 0 after each event is run to allow other threads an opportunity to run in multi-threaded applications.

接著我們來看一個簡單範例, 在 5秒, 10秒 後列印時間 :

>>> import sched, time
>>> s = sched.scheduler(time.time, time.sleep)
>>> def print_time(): print("\t[Info] From print_time: {0}".format(time.time()))
...
>>> def print_some_times():
... print("\t[Info] Now={0}...".format(time.time()))
... s.enter(5, 1, print_time, ()) # 添加五秒後, 執行 print_time() 事件.
... s.enter(10, 1, print_time, ()) # 添加十秒後, 執行 print_time() 事件.
... s.run() # 開始 Event schedule
... print("\t[Info] Now={0}...".format(time.time()))
...
>>> print_some_times()
[Info] Now=1333436870.441...
[Info] From print_time: 1333436875.442
[Info] From print_time: 1333436880.441 # 與前一事件相差五秒
[Info] Now=1333436880.444...

事實上 sched 模組在處理多線程時會有 Thread-safe 的問題, 如果在需要考慮多線程使用情況下, 請使用 threading.Timer. 底下為簡單使用範例 :

>>> import time
>>> from threading import Timer
>>> def print_time():
... print("\t[Info] From print_time: {0}".format(time.time()))
...
>>> def print_some_times():
... print("\t[Info] Now : {0}".format(time.time()))
... Timer(5, print_time, ()).start() # 開始 Thread-1, 在 5 秒後呼叫 print_time()
... Timer(10, print_time, ()).start() # 開始 Thread-2, 在 10 秒後呼叫 print_time()
... time.sleep(11) # Sleep while time-delay events execute
... print("\t[Info] Now : {0}".format(time.time()))
...
>>> print_some_times()
[Info] Now : 1333437800.668
[Info] From print_time: 1333437805.669
[Info] From print_time: 1333437810.669
[Info] Now : 1333437811.669

Scheduler Objects :
scheduler 物件提供下面函數使用 :
- scheduler.enterabs(time, priority, action, argument)

Schedule a new event. The time argument should be a numeric type compatible with the return value of the timefunc function passed to the constructor. Events scheduled for the same time will be executed in the order of their priority.

Executing the event means executing action(*argument). argument must be a sequence holding the parameters for action.

Return value is an event which may be used for later cancellation of the event (see cancel()).

- scheduler.enter(delay, priority, action, argument)

Schedule an event for delay more time units. Other than the relative time, the other arguments, the effect and the return value are the same as those forenterabs().

- scheduler.cancel(event)

Remove the event from the queue. If event is not an event currently in the queue, this method will raise a ValueError.

- scheduler.empty()

Return true if the event queue is empty.

- scheduler.run()

Run all scheduled events. This function will wait (using the delayfunc() function passed to the constructor) for the next event, then execute it and so on until there are no more scheduled events.

Either action or delayfunc can raise an exception. In either case, the scheduler will maintain a consistent state and propagate the exception. If an exception is raised by action, the event will not be attempted in future calls to run().

If a sequence of events takes longer to run than the time available before the next event, the scheduler will simply fall behind. No events will be dropped; the calling code is responsible for canceling events which are no longer pertinent.

- scheduler.queue

New in version 2.6.
Read-only attribute returning a list of upcoming events in the order they will be run. Each event is shown as a named tuple with the following fields: time, priority, action, argument.