Executing tasks in parallel in python

Question

I am using python 2.7, I have some code that looks like this:

task1()
task2()
task3()
dependent1()

task4()
task5()
task6()
dependent2()

dependent3()

The only dependencies here are as follows: dependent1 needs to wait for tasks1-3, dependent2 needs to wait for tasks 4-6 and dependent3 needs to wait for dependents1-2... The following would be okay: running the whole 6 tasks first in parallel, then the first two dependents in parallel.. then the final dependent

I prefer to have as much tasks as possible running in parallel, I've googled for some modules but I was hoping to avoid external libraries, and not sure how the Queue-Thread technique can solve my problem (maybe someone can recommend a good resource?)

Answer 1

The builtin threading.Thread class offers all you need: start to start a new thread and join to wait for the end of a thread.

import threading

def task1():
    pass
def task2():
    pass
def task3():
    pass
def task4():
    pass
def task5():
    pass
def task6():
    pass

def dep1():
    t1 = threading.Thread(target=task1)
    t2 = threading.Thread(target=task2)
    t3 = threading.Thread(target=task3)

    t1.start()
    t2.start()
    t3.start()

    t1.join()
    t2.join()
    t3.join()

def  dep2():
    t4 = threading.Thread(target=task4)
    t5 = threading.Thread(target=task5)

    t4.start()
    t5.start()

    t4.join()
    t5.join()

def dep3():
    d1 = threading.Thread(target=dep1)
    d2 = threading.Thread(target=dep2)

    d1.start()
    d2.start()

    d1.join()
    d2.join()

d3 = threading.Thread(target=dep3)
d3.start()
d3.join()

Alternatively to join you can use Queue.join to wait for the threads end.