The join()
method, when used with threading
or multiprocessing
, is not related to str.join()
– it’s not actually concatenating anything together. Rather, it just means “wait for this [thread/process] to complete”. The name join
is used because the multiprocessing
module’s API is meant to look as similar to the threading
module’s API, and the threading
module uses join
for its Thread
object. Using the term join
to mean “wait for a thread to complete” is common across many programming languages, so Python just adopted it as well.
Now, the reason you see the 20 second delay both with and without the call to join()
is because by default, when the main process is ready to exit, it will implicitly call join()
on all running multiprocessing.Process
instances. This isn’t as clearly stated in the multiprocessing
docs as it should be, but it is mentioned in the Programming Guidelines section:
Remember also that non-daemonic processes will be automatically be
joined.
You can override this behavior by setting the daemon
flag on the Process
to True
prior to starting the process:
p = Process(target=say_hello)
p.daemon = True
p.start()
# Both parent and child will exit here, since the main process has completed.
If you do that, the child process will be terminated as soon as the main process completes:
daemon
The process’s daemon flag, a Boolean value. This must be set before
start() is called.The initial value is inherited from the creating process.
When a process exits, it attempts to terminate all of its daemonic
child processes.