What is the use of join in Python threading

Question

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

User · Answer

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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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  gt  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

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There are a few reasons for the main thread  or any other thread  to join other threads   A thread may have created or holding  locking  some resources  The join-calling thread may be able to clear the resources on its behalf 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

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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

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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  -- gt   You are loitering  Hey  I do not want to loiter  You are not loitering anymore   After 5 seconds -- gt  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   -- gt  You are loitering  You are not loitering anymore   After 5 seconds -- gt  This statement will be printed Hey  I do not want to loiter

User · Answer

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

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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

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With join - interpreter will wait until your process get completed or terminated  gt  gt  gt  from threading import Thread  gt  gt  gt  import time  gt  gt  gt  def sam          print  started        time sleep 10        print  waiting for 10sec        gt  gt  gt  t   Thread target sam   gt  gt  gt  t start   started   gt  gt  gt  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  gt  gt  gt  done   without join - interpreter wont wait until process get terminated   gt  gt  gt  t   Thread target sam   gt  gt  gt  t start   started  gt  gt  gt  print  yes done   without join interpreter wont wait until process get terminated yes done  gt  gt  gt  waiting for 10sec

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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

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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

[python] What is the use of join() in Python threading?

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