Python multithreading broken

Question

I wanted to start learning about threading in python so I grabbed myself a tutorial and started reading and coding. The following code is what I came up with.
I created a function wait that is suppoesed to print out "Waiting started" and then go to sleep for 3 seconds befor printing out "Waiting ended". I created a main function, where I create and start the thread t1. After the thread t1 started I would expect "Meanwile…" to be printed out. However the thread behaves like I initialized it with .run().

Can someone explain this behavior to me and maybe give me a solution how to fix this?

import threading
from time import sleep


def wait():
    print("Waiting started")
    sleep(3)
    print("Waiting ended")


def main():
    t1 = threading.Thread(target=wait())
    t1.start()
    print("Meanwhile...")


if __name__ == '__main__':
    main()

I tried .start() and .run() and for some reason in both cases the output is the same, even though they have a completely diffrent functionality.

Asked By: Niclas17

||

Source

Answer 1

The error is that you call the function wait in line t1 = threading.Thread(target=wait())

The target argument has to be set to the name of the function without brackets. So t1 = threading.Thread(target=wait) should work.

Answered By: TimoGH

Answer 2

I think your problem is that you have written:

t1 = threading.Thread(target=wait())

When you need to write:

t1 = threading.Thread(target=wait)

You need the function without parentheses because the former calls wait() and passes what it returns (nothing) into threading.Thread, while the latter passes the function into threading.Thread, so that it can be called later.

Full code:

import threading
from time import sleep


def wait():
    print("Waiting started")
    sleep(3)
    print("Waiting ended")


def main():
    t1 = threading.Thread(target=wait)
    t1.start()
    print("Meanwhile...")


if __name__ == '__main__':
    main()

Answered By: sbottingota

Python multithreading broken

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