callback_additional_info.py

def apply_async(func, args, *, callback):
    result = func(*args)
    
    callback(result)
    
def add(x, y):
    return x + y

# Using closure
def make_handler():
    sequence = 0
    def handler(result):
        nonlocal sequence
        sequence += 1
        print('[{}] Got: {}'.format(sequence, result))
    return handler

handler = make_handler()
apply_async(add, (2, 3), callback=handler)  # [1] Got: 5
apply_async(add, ('hello', 'world'), callback=handler)  # [2] Got: helloworld