Process.join | Pythontic.com

Method Name:

join

Method Signature:

join()

Return Value:

NoneType

Parameters:

Not Applicable

Method Overview:

Calling join() on a Process instance makes the calling process to wait for the corresponding process.
Once the process is complete the called process proceeds.
In the examples provided on multiprocessing it is the main process that calls the join() method on the Process instances corresponding the child processes.
Calling join() on the Process instance for the same process results in deadlock.
join() can be called only on an already started process.
join() is similar to a person waiting for another person to complete something and proceeding after it.

Example:

import multiprocessing as multiproc

def processCategory1(mq):
mq.put("Category1 message")

def processCategory2(mq):
mq.put("Category2 message")

def processCategory3(mq):
mq.put("Category3 message")

if __name__=="__main__":
print("Parent process started")
multiproc.set_start_method = "forkserver"

mq = multiproc.Queue()

# Create child processes
childProcess1 = multiproc.Process(target = processCategory1, args=(mq,), name="Cat1")
childProcess2 = multiproc.Process(target = processCategory2, args=(mq,), name="Cat2")
childProcess3 = multiproc.Process(target = processCategory3, args=(mq,), name="Cat3")

# Start all the child processes
childProcess1.start()
childProcess2.start()
childProcess3.start()

print("Child processes started....main process waiting for children to complete")

# Wait for all the child processes to complete
childProcess1.join()
childProcess2.join()
childProcess3.join()

print("Child processes completed execution")

while mq.empty() is False:
print(mq.get())

print("Parent process ending")

Output:

Parent process started

Child processes started....main process waiting for children to complete

Child processes completed execution

Category1 message

Category2 message

Category3 message

Parent process ending