Why aren't my thread locks working properly?

Question

For an assignment, I'm supposed to create a simple multi-threaded program that has three threads that wait for a random amount of time (between.1 and 2 seconds) and then print "end." I'm trying to use locks to prevent switching from messing up the output, but my output is still seemingly random (meaning the locks aren't working as intended). My code is:

import time
from threading import *
import random

lock = Lock()

def one():
    global lock
    time.sleep((random.randint(1,20))/10)
    lock.acquire()
    print("1. end")
    lock.release()

def two():
    global lock
    time.sleep((random.randint(1,20))/10)
    lock.acquire()
    print("2. end")
    lock.release()

def three():
    global lock
    time.sleep((random.randint(1,20))/10)
    lock.acquire()
    print("3. end")
    lock.release()


for i in range(0,100):
    t1 = Thread(target = one)
    t2 = Thread(target = two)
    t3 = Thread(target = three)
    t1.start()
    t2.start()
    t3.start()
    t1.join()
    t2.join()
    t3.join()

but my output is:

 2. end
 3. end
 1. end
 3. end
 2. end
 2. end
 1. end
 3. end
 1. end
 etc...

what am I doing wrong?

Answer 1

You acquire the lock after random sleep:

import time
from threading import *
import random

lock = Lock()

def one():
    global lock
    lock.acquire()
    time.sleep((random.randint(1,20))/10)
    print("1. end")
    lock.release()

def two():
    global lock
    lock.acquire()
    time.sleep((random.randint(1,20))/10)
    print("2. end")
    lock.release()

def three():
    global lock
    lock.acquire()
    time.sleep((random.randint(1,20))/10)
    print("3. end")
    lock.release()


for i in range(0,100):
    t1 = Thread(target = one)
    t2 = Thread(target = two)
    t3 = Thread(target = three)
    t1.start()
    t2.start()
    t3.start()
    t1.join()
    t2.join()
    t3.join()