How to thread lock 2 simple threads in Python-CodePudding

I am trying to demonstrate thread locking using a simple example, using lock and release on 2 threads that simply print something out. I am obviously missing something fundamental about how locks work as i cannot understand why it doesn't work.

I have tried too many variations to list here but the main ones are: removing time.sleep(), passing the lock to the threads as an argument.

import threading
import time

thread_lock = threading.Lock()

ctr = 0

def thread_1():
    while True:
        global ctr
        thread_lock.acquire()
        ctr  = 1
        print(ctr, 'thread_1 has added 1 to ctr')
        time.sleep(1)
        thread_lock.release()

def thread_2():
    while True:
        global ctr
        thread_lock.acquire()
        ctr  = 1
        print(ctr, 'thread_2 has added 1 to ctr')
        time.sleep(1)
        thread_lock.release()

t_1 = threading.Thread(target=thread_1)
t_2 = threading.Thread(target=thread_2)

t_1.start()
t_2.start()

# t_1.join()
# t_2.join()

CodePudding user response：

The code below demonstrates how using two locks usually results in the two threads alternating:

import threading
import time

lock1 = threading.Lock()
lock2 = threading.Lock()

ctr = 0
run = True

def thread_1():
    global ctr
    print('thread_1')
    while run:
        lock1.acquire()
        time.sleep(0.001)
        lock2.acquire()
        ctr  = 1
        print(ctr, 'thread_1 has added 1 to ctr')
        time.sleep(1)
        lock1.release()
        time.sleep(0.001)
        lock2.release()
        time.sleep(0.001)

def thread_2():
    global ctr
    print('thread_2')
    time.sleep(0.001)
    while run:
        lock1.acquire()
        time.sleep(0.001)
        lock2.acquire()
        ctr  = 1
        print(ctr, 'thread_2 has added 1 to ctr')
        time.sleep(1)
        lock1.release()
        time.sleep(0.001)
        lock2.release()
        time.sleep(0.001)

t_1 = threading.Thread(target=thread_1)
t_2 = threading.Thread(target=thread_2)

t_1.start()
t_2.start()

while ctr < 5:
    time.sleep(0.1)
    
run = False
print('joining...')
t_1.join()
t_2.join()

Usual output:

thread_1
thread_2
1 thread_1 has added 1 to ctr
2 thread_2 has added 1 to ctr
3 thread_1 has added 1 to ctr
4 thread_2 has added 1 to ctr
5 thread_1 has added 1 to ctr
joining...
6 thread_2 has added 1 to ctr