Python: Pass or Sleep for long running processes?

Question

I am writing an queue processing application which uses threads for waiting on and responding to queue messages to be delivered to the app. For the main part of the applica

Answer 1

If you are looking for a short, zero-cpu way to loop forever until a KeyboardInterrupt, you can use:

from threading import Event

Event().wait()

Note: Due to a bug, this only works on Python 3.2+. In addition, it appears to not work on Windows. For this reason, while True: sleep(1) might be the better option.

For some background, Event objects are normally used for waiting for long running processes to complete:

def do_task():
    sleep(10)
    print('Task complete.')
    event.set()

event = Event()
Thread(do_task).start()
event.wait()

print('Continuing...')

Which prints:

Task complete.
Continuing...

Answer 2

You don't give much context to what you are really doing, but maybe Queue could be used instead of an explicit busy-wait loop? If not, I would assume sleep would be preferable, as I believe it will consume less CPU (as others have already noted).

[Edited according to additional information in comment below.]

Maybe this is obvious, but anyway, what you could do in a case where you are reading information from blocking sockets is to have one thread read from the socket and post suitably formatted messages into a Queue, and then have the rest of your "worker" threads reading from that queue; the workers will then block on reading from the queue without the need for neither pass, nor sleep.

Answer 3

Running a method as a background thread with sleep in Python

    import threading
    import time


    class ThreadingExample(object):
        """ Threading example class
        The run() method will be started and it will run in the background
        until the application exits.
        """

        def __init__(self, interval=1):
            """ Constructor
            :type interval: int
            :param interval: Check interval, in seconds
            """
            self.interval = interval

            thread = threading.Thread(target=self.run, args=())
            thread.daemon = True                            # Daemonize thread
            thread.start()                                  # Start the execution

        def run(self):
            """ Method that runs forever """
            while True:
                # Do something
                print('Doing something imporant in the background')

                time.sleep(self.interval)

    example = ThreadingExample()
    time.sleep(3)
    print('Checkpoint')
    time.sleep(2)
    print('Bye')

Answer 4

I've always seen/heard that using sleep is the better way to do it. Using sleep will keep your Python interpreter's CPU usage from going wild.

Answer 5

I would imagine time.sleep() will have less overhead on the system. Using pass will cause the loop to immediately re-evaluate and peg the CPU, whereas using time.sleep will allow the execution to be temporarily suspended.

EDIT: just to prove the point, if you launch the python interpreter and run this:

>>> while True:
...     pass
... 

You can watch Python start eating up 90-100% CPU instantly, versus:

>>> import time 
>>> while True:
...     time.sleep(1)
... 

Which barely even registers on the Activity Monitor (using OS X here but it should be the same for every platform).

Answer 6

signal.pause() is another solution, see https://docs.python.org/3/library/signal.html#signal.pause

Cause the process to sleep until a signal is received; the appropriate handler will then be called. Returns nothing. Not on Windows. (See the Unix man page signal(2).)