问题:Python线程中的join()有什么用?

我正在研究python线程并遇到了join()

作者告诉我们,如果线程处于守护程序模式,那么我需要使用它,join()以便线程可以在主线程终止之前完成自身。

但我一直在使用也没见他t.join()即使tdaemon

示例代码是这个

import threading
import time
import logging

logging.basicConfig(level=logging.DEBUG,
                    format='(%(threadName)-10s) %(message)s',
                    )

def daemon():
    logging.debug('Starting')
    time.sleep(2)
    logging.debug('Exiting')

d = threading.Thread(name='daemon', target=daemon)
d.setDaemon(True)

def non_daemon():
    logging.debug('Starting')
    logging.debug('Exiting')

t = threading.Thread(name='non-daemon', target=non_daemon)

d.start()
t.start()

d.join()
t.join()

我不知道这是什么用途,t.join()因为它不是守护程序,即使删除它也看不到任何变化

I was studying the python threading and came across join().

The author told that if thread is in daemon mode then i need to use join() so that thread can finish itself before main thread terminates.

but I have also seen him using t.join() even though t was not daemon

example code is this

import threading
import time
import logging

logging.basicConfig(level=logging.DEBUG,
                    format='(%(threadName)-10s) %(message)s',
                    )

def daemon():
    logging.debug('Starting')
    time.sleep(2)
    logging.debug('Exiting')

d = threading.Thread(name='daemon', target=daemon)
d.setDaemon(True)

def non_daemon():
    logging.debug('Starting')
    logging.debug('Exiting')

t = threading.Thread(name='non-daemon', target=non_daemon)

d.start()
t.start()

d.join()
t.join()

i don’t know what is use of t.join() as it is not daemon and i can see no change even if i remove it


回答 0

展示这种机制的技术有些笨拙:join()大概是由主线程调用的。也可以由另一个线程调用它,但是会不必要地使该图复杂化。

join-调用应该放在主线程的轨道上,但是为了表达线程关系并使其尽可能简单,我选择将其放在子线程中。

without join:
+---+---+------------------                     main-thread
    |   |
    |   +...........                            child-thread(short)
    +..................................         child-thread(long)

with join
+---+---+------------------***********+###      main-thread
    |   |                             |
    |   +...........join()            |         child-thread(short)
    +......................join()......         child-thread(long)

with join and daemon thread
+-+--+---+------------------***********+###     parent-thread
  |  |   |                             |
  |  |   +...........join()            |        child-thread(short)
  |  +......................join()......        child-thread(long)
  +,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,     child-thread(long + daemonized)

'-' main-thread/parent-thread/main-program execution
'.' child-thread execution
'#' optional parent-thread execution after join()-blocked parent-thread could 
    continue
'*' main-thread 'sleeping' in join-method, waiting for child-thread to finish
',' daemonized thread - 'ignores' lifetime of other threads;
    terminates when main-programs exits; is normally meant for 
    join-independent tasks

因此,您看不到任何更改的原因是因为您的主线程在之后没有执行任何操作join。您可以说join(仅)与主线程的执行流程相关。

例如,如果要同时下载一堆页面以将它们串联成一个大页面,则可以使用线程开始并发下载,但是需要等到最后一页/线程完成后才能开始组装单个页面在很多。那是您使用的时间join()

A somewhat clumsy ascii-art to demonstrate the mechanism: The join() is presumably called by the main-thread. It could also be called by another thread, but would needlessly complicate the diagram.

join-calling should be placed in the track of the main-thread, but to express thread-relation and keep it as simple as possible, I choose to place it in the child-thread instead.

without join:
+---+---+------------------                     main-thread
    |   |
    |   +...........                            child-thread(short)
    +..................................         child-thread(long)

with join
+---+---+------------------***********+###      main-thread
    |   |                             |
    |   +...........join()            |         child-thread(short)
    +......................join()......         child-thread(long)

with join and daemon thread
+-+--+---+------------------***********+###     parent-thread
  |  |   |                             |
  |  |   +...........join()            |        child-thread(short)
  |  +......................join()......        child-thread(long)
  +,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,     child-thread(long + daemonized)

'-' main-thread/parent-thread/main-program execution
'.' child-thread execution
'#' optional parent-thread execution after join()-blocked parent-thread could 
    continue
'*' main-thread 'sleeping' in join-method, waiting for child-thread to finish
',' daemonized thread - 'ignores' lifetime of other threads;
    terminates when main-programs exits; is normally meant for 
    join-independent tasks

So the reason you don’t see any changes is because your main-thread does nothing after your join. You could say join is (only) relevant for the execution-flow of the main-thread.

If, for example, you want to concurrently download a bunch of pages to concatenate them into a single large page, you may start concurrent downloads using threads, but need to wait until the last page/thread is finished before you start assembling a single page out of many. That’s when you use join().


回答 1

直接来自文档

join([timeout])等待线程终止。这将阻塞调用线程,直到被调用join()方法的线程终止(正常或通过未处理的异常终止),或者直到发生可选的超时。

这意味着,主线程其产卵td,等待t完成,直到它完成。

根据您的程序采用的逻辑,您可能要等到线程完成后再继续执行主线程。

另外从文档:

线程可以标记为“守护程序线程”。该标志的重要性在于,仅保留守护程序线程时,整个Python程序都会退出。

一个简单的例子,说我们有这个:

def non_daemon():
    time.sleep(5)
    print 'Test non-daemon'

t = threading.Thread(name='non-daemon', target=non_daemon)

t.start()

结束于:

print 'Test one'
t.join()
print 'Test two'

这将输出:

Test one
Test non-daemon
Test two

在此,主线程显式等待t线程完成,直到print第二次调用为止。

或者,如果我们有这个:

print 'Test one'
print 'Test two'
t.join()

我们将得到以下输出:

Test one
Test two
Test non-daemon

在这里,我们在主线程中完成工作,然后等待t线程完成。在这种情况下,我们甚至可以删除显式连接t.join(),并且程序将隐式等待t完成。

Straight from the docs

join([timeout]) Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates – either normally or through an unhandled exception – or until the optional timeout occurs.

This means that the main thread which spawns t and d, waits for t to finish until it finishes.

Depending on the logic your program employs, you may want to wait until a thread finishes before your main thread continues.

Also from the docs:

A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left.

A simple example, say we have this:

def non_daemon():
    time.sleep(5)
    print 'Test non-daemon'

t = threading.Thread(name='non-daemon', target=non_daemon)

t.start()

Which finishes with:

print 'Test one'
t.join()
print 'Test two'

This will output:

Test one
Test non-daemon
Test two

Here the master thread explicitly waits for the t thread to finish until it calls print the second time.

Alternatively if we had this:

print 'Test one'
print 'Test two'
t.join()

We’ll get this output:

Test one
Test two
Test non-daemon

Here we do our job in the main thread and then we wait for the t thread to finish. In this case we might even remove the explicit joining t.join() and the program will implicitly wait for t to finish.


回答 2

感谢您提供此主题-它对我也有很大帮助。

我今天学到了有关.join()的知识。

这些线程并行运行:

d.start()
t.start()
d.join()
t.join()

这些依次运行(不是我想要的):

d.start()
d.join()
t.start()
t.join()

特别是,我试图变得聪明和整洁:

class Kiki(threading.Thread):
    def __init__(self, time):
        super(Kiki, self).__init__()
        self.time = time
        self.start()
        self.join()

这可行!但是它顺序运行。我可以将self.start()放在__ init __中,但是不能将self.join()放入。必须每个线程启动之后执行此操作。

join()是导致主线程等待线程完成的原因。否则,您的线程将独自运行。

因此,可以将join()视为主线程上的一个“保留”方法-它可以对线程进行解线程,并在主线程可以继续之前在主线程中按顺序执行。它可以确保您的线程在主线程向前移动之前是完整的。请注意,这意味着在调用join()之前线程已经完成是可以的-在调用join()时立即立即释放主线程。

实际上,我刚才想到的是,主线程在d.join()上等待,直到线程d完成,然后才移至t.join()。

实际上,非常清楚,请考虑以下代码:

import threading
import time

class Kiki(threading.Thread):
    def __init__(self, time):
        super(Kiki, self).__init__()
        self.time = time
        self.start()

    def run(self):
        print self.time, " seconds start!"
        for i in range(0,self.time):
            time.sleep(1)
            print "1 sec of ", self.time
        print self.time, " seconds finished!"


t1 = Kiki(3)
t2 = Kiki(2)
t3 = Kiki(1)
t1.join()
print "t1.join() finished"
t2.join()
print "t2.join() finished"
t3.join()
print "t3.join() finished"

它产生此输出(请注意print语句如何彼此穿线。)

$ python test_thread.py
32   seconds start! seconds start!1

 seconds start!
1 sec of  1
 1 sec of 1  seconds finished!
 21 sec of
3
1 sec of  3
1 sec of  2
2  seconds finished!
1 sec of  3
3  seconds finished!
t1.join() finished
t2.join() finished
t3.join() finished
$ 

t1.join()正在阻止主线程。所有三个线程在t1.join()完成之前完成,并且主线程继续执行打印,然后执行t2.join()然后执行打印,然后执行t3.join()然后进行打印。

欢迎更正。我也是线程新手。

(注意:如果您有兴趣,我正在为DrinkBot编写代码,并且我需要线程来同时运行配料泵,而不是顺序运行-等待每杯饮料的时间更少。)

Thanks for this thread — it helped me a lot too.

I learned something about .join() today.

These threads run in parallel:

d.start()
t.start()
d.join()
t.join()

and these run sequentially (not what I wanted):

d.start()
d.join()
t.start()
t.join()

In particular, I was trying to clever and tidy:

class Kiki(threading.Thread):
    def __init__(self, time):
        super(Kiki, self).__init__()
        self.time = time
        self.start()
        self.join()

This works! But it runs sequentially. I can put the self.start() in __ init __, but not the self.join(). That has to be done after every thread has been started.

join() is what causes the main thread to wait for your thread to finish. Otherwise, your thread runs all by itself.

So one way to think of join() as a “hold” on the main thread — it sort of de-threads your thread and executes sequentially in the main thread, before the main thread can continue. It assures that your thread is complete before the main thread moves forward. Note that this means it’s ok if your thread is already finished before you call the join() — the main thread is simply released immediately when join() is called.

In fact, it just now occurs to me that the main thread waits at d.join() until thread d finishes before it moves on to t.join().

In fact, to be very clear, consider this code:

import threading
import time

class Kiki(threading.Thread):
    def __init__(self, time):
        super(Kiki, self).__init__()
        self.time = time
        self.start()

    def run(self):
        print self.time, " seconds start!"
        for i in range(0,self.time):
            time.sleep(1)
            print "1 sec of ", self.time
        print self.time, " seconds finished!"


t1 = Kiki(3)
t2 = Kiki(2)
t3 = Kiki(1)
t1.join()
print "t1.join() finished"
t2.join()
print "t2.join() finished"
t3.join()
print "t3.join() finished"

It produces this output (note how the print statements are threaded into each other.)

$ python test_thread.py
32   seconds start! seconds start!1

 seconds start!
1 sec of  1
 1 sec of 1  seconds finished!
 21 sec of
3
1 sec of  3
1 sec of  2
2  seconds finished!
1 sec of  3
3  seconds finished!
t1.join() finished
t2.join() finished
t3.join() finished
$ 

The t1.join() is holding up the main thread. All three threads complete before the t1.join() finishes and the main thread moves on to execute the print then t2.join() then print then t3.join() then print.

Corrections welcome. I’m also new to threading.

(Note: in case you’re interested, I’m writing code for a DrinkBot, and I need threading to run the ingredient pumps concurrently rather than sequentially — less time to wait for each drink.)


回答 3

方法join()

阻塞调用线程,直到调用join()方法的线程终止。

来源:http : //docs.python.org/2/library/threading.html

The method join()

blocks the calling thread until the thread whose join() method is called is terminated.

Source : http://docs.python.org/2/library/threading.html


回答 4

简单了解

与join-解释器将等待,直到您的过程完成终止

>>> from threading import Thread
>>> import time
>>> def sam():
...   print 'started'
...   time.sleep(10)
...   print 'waiting for 10sec'
... 
>>> t = Thread(target=sam)
>>> t.start()
started

>>> t.join() # with join interpreter will wait until your process get completed or terminated
done?   # this line printed after thread execution stopped i.e after 10sec
waiting for 10sec
>>> done?

没有连接解释器不会等到进程被终止

>>> t = Thread(target=sam)
>>> t.start()
started
>>> print 'yes done' #without join interpreter wont wait until process get terminated
yes done
>>> waiting for 10sec

With join – interpreter will wait until your process get completed or terminated

>>> from threading import Thread
>>> import time
>>> def sam():
...   print 'started'
...   time.sleep(10)
...   print 'waiting for 10sec'
... 
>>> t = Thread(target=sam)
>>> t.start()
started

>>> t.join() # with join interpreter will wait until your process get completed or terminated
done?   # this line printed after thread execution stopped i.e after 10sec
waiting for 10sec
>>> done?

without join – interpreter wont wait until process get terminated,

>>> t = Thread(target=sam)
>>> t.start()
started
>>> print 'yes done' #without join interpreter wont wait until process get terminated
yes done
>>> waiting for 10sec

回答 5

join(t)同时为非守护程序线程和守护程序线程创建函数时,主线程(或主进程)应等待t几秒钟,然后才能进一步在其自己的进程上工作。在t几秒钟的等待时间内,两个子线程都应该做他们可以做的事情,例如打印一些文本。在t几秒钟之后,如果非守护程序线程仍然没有完成其工作,并且它仍可以在主进程完成其工作之后完成它,但是对于守护程序线程,它只是错过了机会窗口。但是,它将在python程序退出后最终消失。如果有问题,请纠正我。

When making join(t) function for both non-daemon thread and daemon thread, the main thread (or main process) should wait t seconds, then can go further to work on its own process. During the t seconds waiting time, both of the children threads should do what they can do, such as printing out some text. After the t seconds, if non-daemon thread still didn’t finish its job, and it still can finish it after the main process finishes its job, but for daemon thread, it just missed its opportunity window. However, it will eventually die after the python program exits. Please correct me if there is something wrong.


回答 6

在python 3.x中,join()用于将线程与主线程连接,即,当join()用于特定线程时,主线程将停止执行,直到连接线程的执行完成为止。

#1 - Without Join():
import threading
import time
def loiter():
    print('You are loitering!')
    time.sleep(5)
    print('You are not loitering anymore!')

t1 = threading.Thread(target = loiter)
t1.start()
print('Hey, I do not want to loiter!')
'''
Output without join()--> 
You are loitering!
Hey, I do not want to loiter!
You are not loitering anymore! #After 5 seconds --> This statement will be printed

'''
#2 - With Join():
import threading
import time
def loiter():
    print('You are loitering!')
    time.sleep(5)
    print('You are not loitering anymore!')

t1 = threading.Thread(target = loiter)
t1.start()
t1.join()
print('Hey, I do not want to loiter!')

'''
Output with join() -->
You are loitering!
You are not loitering anymore! #After 5 seconds --> This statement will be printed
Hey, I do not want to loiter! 

'''

In python 3.x join() is used to join a thread with the main thread i.e. when join() is used for a particular thread the main thread will stop executing until the execution of joined thread is complete.

#1 - Without Join():
import threading
import time
def loiter():
    print('You are loitering!')
    time.sleep(5)
    print('You are not loitering anymore!')

t1 = threading.Thread(target = loiter)
t1.start()
print('Hey, I do not want to loiter!')
'''
Output without join()--> 
You are loitering!
Hey, I do not want to loiter!
You are not loitering anymore! #After 5 seconds --> This statement will be printed

'''
#2 - With Join():
import threading
import time
def loiter():
    print('You are loitering!')
    time.sleep(5)
    print('You are not loitering anymore!')

t1 = threading.Thread(target = loiter)
t1.start()
t1.join()
print('Hey, I do not want to loiter!')

'''
Output with join() -->
You are loitering!
You are not loitering anymore! #After 5 seconds --> This statement will be printed
Hey, I do not want to loiter! 

'''

回答 7

此示例演示了该.join()操作:

import threading
import time

def threaded_worker():
    for r in range(10):
        print('Other: ', r)
        time.sleep(2)

thread_ = threading.Timer(1, threaded_worker)
thread_.daemon = True  # If the main thread kills, this thread will be killed too. 
thread_.start()

flag = True

for i in range(10):
    print('Main: ', i)
    time.sleep(2)
    if flag and i > 4:
        print(
            '''
            Threaded_worker() joined to the main thread. 
            Now we have a sequential behavior instead of concurrency.
            ''')
        thread_.join()
        flag = False

出:

Main:  0
Other:  0
Main:  1
Other:  1
Main:  2
Other:  2
Main:  3
Other:  3
Main:  4
Other:  4
Main:  5
Other:  5

            Threaded_worker() joined to the main thread. 
            Now we have a sequential behavior instead of concurrency.

Other:  6
Other:  7
Other:  8
Other:  9
Main:  6
Main:  7
Main:  8
Main:  9

This example demonstrate the .join() action:

import threading
import time

def threaded_worker():
    for r in range(10):
        print('Other: ', r)
        time.sleep(2)

thread_ = threading.Timer(1, threaded_worker)
thread_.daemon = True  # If the main thread kills, this thread will be killed too. 
thread_.start()

flag = True

for i in range(10):
    print('Main: ', i)
    time.sleep(2)
    if flag and i > 4:
        print(
            '''
            Threaded_worker() joined to the main thread. 
            Now we have a sequential behavior instead of concurrency.
            ''')
        thread_.join()
        flag = False

Out:

Main:  0
Other:  0
Main:  1
Other:  1
Main:  2
Other:  2
Main:  3
Other:  3
Main:  4
Other:  4
Main:  5
Other:  5

            Threaded_worker() joined to the main thread. 
            Now we have a sequential behavior instead of concurrency.

Other:  6
Other:  7
Other:  8
Other:  9
Main:  6
Main:  7
Main:  8
Main:  9

回答 8

主线程(或任何其他线程)加入其他线程的原因有几个

  1. 线程可能已经创建或持有(锁定)了一些资源。加入调用线程可能能够代表它清除资源

  2. join()是自然的阻塞调用,用于在被调用线程终止后继续继续调用join调用线程。

如果python程序未加入其他线程,则python解释器仍将代表其加入非守护程序线程。

There are a few reasons for the main thread (or any other thread) to join other threads

  1. A thread may have created or holding (locking) some resources. The join-calling thread may be able to clear the resources on its behalf

  2. join() is a natural blocking call for the join-calling thread to continue after the called thread has terminated.

If a python program does not join other threads, the python interpreter will still join non-daemon threads on its behalf.


回答 9

“使用join()有什么用?” 你说。确实,答案与“关闭文件有什么用,因为当程序退出时python和OS会为我关闭我的文件吗?”。

这仅仅是一个良好的编程问题。您应该在代码中不应运行该线程的位置加入join()线程,这是因为您肯定要确保该线程未在运行以干扰您自己的代码,或者您想在一个线程中正确运行更大的系统。

您可能因为join()可能需要额外的时间而说“我不希望我的代码延迟给出答案”。在某些情况下,这可能是完全正确的,但是现在您需要考虑到您的代码“为Python和OS清理留下了麻烦”。如果出于性能原因执行此操作,强烈建议您记录该行为。如果您要构建其他人希望使用的库/包,则尤其如此。

除了性能原因外,没有任何理由不加入join(),我认为您的代码不需要表现得那么好。

“What’s the use of using join()?” you say. Really, it’s the same answer as “what’s the use of closing files, since python and the OS will close my file for me when my program exits?”.

It’s simply a matter of good programming. You should join() your threads at the point in the code that the thread should not be running anymore, either because you positively have to ensure the thread is not running to interfere with your own code, or that you want to behave correctly in a larger system.

You might say “I don’t want my code to delay giving an answer” just because of the additional time that the join() might require. This may be perfectly valid in some scenarios, but you now need to take into account that your code is “leaving cruft around for python and the OS to clean up”. If you do this for performance reasons, I strongly encourage you to document that behavior. This is especially true if you’re building a library/package that others are expected to utilize.

There’s no reason to not join(), other than performance reasons, and I would argue that your code does not need to perform that well.


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