Why is my multiprocessing in python not performing the task?

Question

I want to put the function abc running in parallel, and the task ppm.abc in the apply_async doesn't perform and I'm only getting the error_calback value.

I've tried to run the function outside the apply_async, and it runs perfectly. Does anyone know what the problem is?Here is the code

if __name__ == "__main__":
    start_time = time.time()  
    pool = multiprocessing.Pool(processes=multiprocessing.cpu_count()-1)
    ppm=PPM()
    for p in range(len(routes)):
        pool.apply_async(ppm.abc,args=(p), callback=ppm.collect_results,error_callback=ppm.custom_error_callback)
    pool.close()
    pool.join()