import json
from concurrent.futures import \
    ThreadPoolExecutor, as_completed

# task that is performed in parallel
def task(index):     
    return ['processes', index], index

def lambda_handler(event, context):
    # TODO implement
    all_tasks = []
    list_of_items = [i for i in range(20)]
    with ThreadPoolExecutor(max_workers=4) as executor:
        for item_index in range(len(list_of_items)):
            all_tasks.append(
                executor.submit(
                    task, 
                    item_index)
                )

        temp_res = list(range(len(list_of_items)))
        
        # process completed tasks
        for future in as_completed(all_tasks):
            rs, index = future.result()
            temp_res[index] = rs

        for t in temp_res:
            print("t", t)
    return {
        'statusCode': 200,
        'body': json.dumps('Hello from Lambda!')
    }