Python Threading - Crash with multiple threads

Question

I wrote a small python script using Python's threading module for multi-threading.
The target function that gets called by threading.Thread takes 2 arguments (self and another value). But I always get the following error TypeError: example() takes 2 positional arguments but 3 were given even if only 2 arguments are given.

import threading
import random
num=random.randint(1,999)

threadN=10 #number of processes
a="11" #testing value
class ExampleClass():
    def __init__(self):
        self.num=num

    def example(self,a):
        print(self.num)
        print(a)

if __name__ == '__main__':
    cl=ExampleClass()
    while threadN>0:

        threading.Thread(target=cl.example, args=(a)).start()
        threadN-=1

Any help would be appreciated!

`args` has to be list or tuple but `()` doesn't create tuple. You have to add comma - `args=(a,)` — furas, Aug 20 '19 at 08:36
see https://stackoverflow.com/q/12876177/1358308 for why `(a)` isn't doing what you think it's doing — Sam Mason, Aug 20 '19 at 09:16

score 3 · Accepted Answer · answered Aug 20 '19 at 08:44

3

args has to be list or tuple but () doesn't create tuple. You have to use comma to create tuple with single value - args=(a,)

 threading.Thread(target=cl.example, args=(t,)).start()

() is here only to separate comma which creates tuple from comma which separate arguments in function. You could do the same without ()but you would have to create tuple before thread

 arguments = a,
 threading.Thread(target=cl.example, args=arguments).start()

answered Aug 20 '19 at 08:44

furas

134,197
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interesting! thanks for pointing this out! didn't know tuple was needed, thought either list or just a string. Thanks!! – rocket Aug 20 '19 at 09:21

score 2 · Answer 2 · answered Aug 20 '19 at 08:26

2

Okay, just found my issue, looked again in the docs and found out following thing:

threading.Thread(target=cl.example args=[t]).start() Using [ ] for the arguments solves the problem...

answered Aug 20 '19 at 08:26

rocket

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cant mark it yet, as its my own answer and that means I cannot accept it as answer in less than 2 days. But will do once I can! – rocket Aug 20 '19 at 08:33

Python Threading - Crash with multiple threads

2 Answers2