Update multiprocessing example

[Windsooon]

BPO	34051
Nosy	@pitrou, @zware, @applio, @Windsooon

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/applio'
closed_at = <Date 2018-07-08.04:15:01.805>
created_at = <Date 2018-07-05.08:45:49.470>
labels = ['3.8', 'docs']
title = 'Update multiprocessing example'
updated_at = <Date 2018-07-08.04:15:01.804>
user = 'https://github.com/Windsooon'

bugs.python.org fields:

activity = <Date 2018-07-08.04:15:01.804>
actor = 'Windson Yang'
assignee = 'davin'
closed = True
closed_date = <Date 2018-07-08.04:15:01.805>
closer = 'Windson Yang'
components = ['Documentation']
creation = <Date 2018-07-05.08:45:49.470>
creator = 'Windson Yang'
dependencies = []
files = []
hgrepos = []
issue_num = 34051
keywords = []
message_count = 5.0
messages = ['321088', '321210', '321219', '321220', '321255']
nosy_count = 5.0
nosy_names = ['pitrou', 'docs@python', 'zach.ware', 'davin', 'Windson Yang']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34051'
versions = ['Python 3.8']

[Windsooon]

The docs at https://docs.python.org/3.8/library/multiprocessing.html#synchronization-between-processes give an example:

from multiprocessing import Process, Lock

def f(l, i):
    l.acquire()
    try:
        print('hello world', i)
    finally:
        l.release()

if __name__ == '__main__':
    lock = Lock()

    for num in range(10):
        Process(target=f, args=(lock, num)).start()

and point out "For instance one can use a lock to ensure that only one process prints to standard output at a time...". I'm not sure this is a good enough example for the reader. The reader can't tell the difference between the function with l.acquire() or not, The output just shows in the terminal at the same time. So I think a better idea just add time.sleep(0.1) before print('hello world', i) like this:

    l.acquire()
    try:
        # do something here
        # time.sleep(0.1)
        print('hello world', i)

I can provide a pr if you guys like this idea.

[pitrou]

The example feels a bit artificial indeed, but I don't think adding a sleep() call would make it realistic. Why would you protect sleep() with a lock?

[Windsooon]

Hello, @antoine Pitrou. Maybe there is another way to let the reader know "only one process prints to standard output at a time" instead of sleep() function?

[pitrou]

To be frank, I don't think that matters much. The user should understand what a lock is already, if they want to make use of multiprocessing fruitfully. The example showcases how to create a lock and how to pass it to child processes (by giving it as a function parameter). Printing to standard output is not the important thing here.

However if you want to improve this example, you could replace the acquire/release pair with a "with" statement.

[Windsooon]

Thank you, I think to use acquire() and release() may be better than with statement in this example. I will close this issue.