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What is the use of join() in threading?

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

JavaScript

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

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Answer

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.

JavaScript

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().

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