import threading
import time
import random

# Create a function to do our computationally expensive work.
# We'll just simulate a long calculation with time.sleep
def do_work(number):
    time.sleep(random.random() * 3)
    print(number, 'is done')

# Using a list comprehension, we create a list of 4 threads that each run the
# do_work function and pass in a value for its 'number' parameter.
threads = [threading.Thread(target=do_work, args=(i,)) for i in range(1, 5)]

# Start execution of all the threads in parallel. They'll likely finish out
# of order.
[t.start() for t in threads]
# 1 is done
# 3 is done
# 4 is done
# 2 is done

# The main thread is not blocked while the other threads are running. If you
# want to block the main thread until another thread completes, call join()
# on the other thread.