Pass different arguments to ThreadPool.apply_async() method in python

Question

I want to pass different arguments to ThreadPool.apply_async() method. How I can do it? I thought it was done in the following way, but I was wrong. Could I use Thread instead? Thank you in advance

#!/usr/bin/env python
import multiprocessing

def testMethod(arg1, arg2, arg3):
    print(f"{arg1} - {arg2} - {arg3}"

if __name__ == '__main__':
    pool = multiprocessing.Pool()
    for t in range(1000):
        pool.apply_async(testMethod, t, arg2, arg3)

Answer 1

The type hints on the method signature in typeshed show, that postional arguments must be supplied as an iterable:

def apply_async(self,
                func: (...) -> _T,
                args: Iterable = ...,
                kwds: Mapping[str, Any] = ...,
                callback: Optional[(_T) -> None] = ...,
                error_callback: Optional[(BaseException) -> None] = ...)

Your call to apply_async should look like this:

pool.apply_async(testMethod, args=(t, 'foo', 'bar'))

Or, using a keyword argument mapping:

pool.apply_async(testMethod, kwds={'arg1': t, 'arg2': 'foo', 'arg3': 'bar'})

As an aside: Your code example does not use a thread pool; multiprocessing.Pool is a class that represents a process pool, i.e. this creates seperate processes to act as workers.
If you want to use a thread pool, go for:

from multiprocessing.pool import ThreadPool
pool = ThreadPool()

The interface is similar to that of the process pool.