Example of thread lock in Python programming

Python's threading module provides a variety of lock-related methods. Python's multiple threads cannot be executed at the same time, so the use of locks is very critical. Let's take an example to explain the use of thread locks in Python programming:

Lock

Python's built-in data structures such as lists and dictionaries are thread-safe, but simple data types such as integers and floating-point numbers are not thread-safe. These simple data Type of pass operation requires the use of locks.

#!/usr/bin/env python3
# coding=utf-8

import threading

shared_resource_with_lock = 0
shared_resource_with_no_lock = 0
COUNT = 100000
shared_resource_lock = threading.Lock()

####LOCK MANAGEMENT##
def increment_with_lock():
  global shared_resource_with_lock
  for i in range(COUNT):
    shared_resource_lock.acquire()
    shared_resource_with_lock += 1
    shared_resource_lock.release()
    
def decrement_with_lock():
  global shared_resource_with_lock
  for i in range(COUNT):
    shared_resource_lock.acquire()
    shared_resource_with_lock -= 1
    shared_resource_lock.release()
    ####NO LOCK MANAGEMENT ##
  
def increment_without_lock():
  global shared_resource_with_no_lock
  for i in range(COUNT):
    shared_resource_with_no_lock += 1
  
def decrement_without_lock():
  global shared_resource_with_no_lock
  for i in range(COUNT):
    shared_resource_with_no_lock -= 1
  
####the Main program
if __name__ == "__main__":
  t1 = threading.Thread(target = increment_with_lock)
  t2 = threading.Thread(target = decrement_with_lock)
  t3 = threading.Thread(target = increment_without_lock)
  t4 = threading.Thread(target = decrement_without_lock)
  t1.start()
  t2.start()
  t3.start()
  t4.start()
  t1.join()
  t2.join()
  t3.join()
  t4.join()
  print ("the value of shared variable with lock management is %s"\
  %shared_resource_with_lock)
  print ("the value of shared variable with race condition is %s"\
  %shared_resource_with_no_lock)

Execution result:

$ ./threading_lock.py

the value of shared variable with lock management is 0
the value of shared variable with race condition is 0

Another example:

import random
import threading
import time
logging.basicConfig(level=logging.DEBUG,
          format='(%(threadName)-10s) %(message)s',
          )
          
class Counter(object):
  def __init__(self, start=0):
    self.lock = threading.Lock()
    self.value = start
  def increment(self):
    logging.debug(time.ctime(time.time()))
    logging.debug('Waiting for lock')
    self.lock.acquire()
    try:
      pause = random.randint(1,3)
      logging.debug(time.ctime(time.time()))
      logging.debug('Acquired lock')      
      self.value = self.value + 1
      logging.debug('lock {0} seconds'.format(pause))
      time.sleep(pause)
    finally:
      self.lock.release()
def worker(c):
  for i in range(2):
    pause = random.randint(1,3)
    logging.debug(time.ctime(time.time()))
    logging.debug('Sleeping %0.02f', pause)
    time.sleep(pause)
    c.increment()
  logging.debug('Done')
counter = Counter()
for i in range(2):
  t = threading.Thread(target=worker, args=(counter,))
  t.start()
logging.debug('Waiting for worker threads')
main_thread = threading.currentThread()
for t in threading.enumerate():
  if t is not main_thread:
    t.join()
logging.debug('Counter: %d', counter.value)

Execution result:

$ python threading_lock.py

(Thread-1 ) Tue Sep 15 15:49:18 2015
(Thread-1 ) Sleeping 3.00
(Thread-2 ) Tue Sep 15 15:49:18 2015
(MainThread) Waiting for worker threads
(Thread-2 ) Sleeping 2.00
(Thread-2 ) Tue Sep 15 15:49:20 2015
(Thread-2 ) Waiting for lock
(Thread-2 ) Tue Sep 15 15:49:20 2015
(Thread-2 ) Acquired lock
(Thread-2 ) lock 2 seconds
(Thread-1 ) Tue Sep 15 15:49:21 2015
(Thread-1 ) Waiting for lock
(Thread-2 ) Tue Sep 15 15:49:22 2015
(Thread-1 ) Tue Sep 15 15:49:22 2015
(Thread-2 ) Sleeping 2.00
(Thread-1 ) Acquired lock
(Thread-1 ) lock 1 seconds
(Thread-1 ) Tue Sep 15 15:49:23 2015
(Thread-1 ) Sleeping 2.00
(Thread-2 ) Tue Sep 15 15:49:24 2015
(Thread-2 ) Waiting for lock
(Thread-2 ) Tue Sep 15 15:49:24 2015
(Thread-2 ) Acquired lock
(Thread-2 ) lock 1 seconds
(Thread-1 ) Tue Sep 15 15:49:25 2015
(Thread-1 ) Waiting for lock
(Thread-1 ) Tue Sep 15 15:49:25 2015
(Thread-1 ) Acquired lock
(Thread-1 ) lock 2 seconds
(Thread-2 ) Done
(Thread-1 ) Done
(MainThread) Counter: 4

Passing a False value into acquire() can check whether the lock is acquired. For example:

import logging
import threading
import time
logging.basicConfig(level=logging.DEBUG,
          format='(%(threadName)-10s) %(message)s',
          )
          
def lock_holder(lock):
  logging.debug('Starting')
  while True:
    lock.acquire()
    try:
      logging.debug('Holding')
      time.sleep(0.5)
    finally:
      logging.debug('Not holding')
      lock.release()
    time.sleep(0.5)
  return
          
def worker(lock):
  logging.debug('Starting')
  num_tries = 0
  num_acquires = 0
  while num_acquires < 3:
    time.sleep(0.5)
    logging.debug(&#39;Trying to acquire&#39;)
    have_it = lock.acquire(0)
    try:
      num_tries += 1
      if have_it:
        logging.debug(&#39;Iteration %d: Acquired&#39;,
               num_tries)
        num_acquires += 1
      else:
        logging.debug(&#39;Iteration %d: Not acquired&#39;,
               num_tries)
    finally:
      if have_it:
        lock.release()
  logging.debug(&#39;Done after %d iterations&#39;, num_tries)
lock = threading.Lock()
holder = threading.Thread(target=lock_holder,
             args=(lock,),
             name=&#39;LockHolder&#39;)
holder.setDaemon(True)
holder.start()
worker = threading.Thread(target=worker,
             args=(lock,),
             name=&#39;Worker&#39;)
worker.start()

Execution result:

$ python threading_lock_noblock.py

(LockHolder) Starting
(LockHolder) Holding
(Worker  ) Starting
(LockHolder) Not holding
(Worker  ) Trying to acquire
(Worker  ) Iteration 1: Acquired
(LockHolder) Holding
(Worker  ) Trying to acquire
(Worker  ) Iteration 2: Not acquired
(LockHolder) Not holding
(Worker  ) Trying to acquire
(Worker  ) Iteration 3: Acquired
(LockHolder) Holding
(Worker  ) Trying to acquire
(Worker  ) Iteration 4: Not acquired
(LockHolder) Not holding
(Worker  ) Trying to acquire
(Worker  ) Iteration 5: Acquired
(Worker  ) Done after 5 iterations

Thread-safe lock

threading.RLock()

Returns a reentrant lock object. A reentrant lock must be released by the thread that acquired it. Once a thread acquires a reentrant lock, the same thread can acquire it again without blocking, and must be released after acquisition.

Usually a thread can only acquire the lock once:

import threading

lock = threading.Lock()

print 'First try :', lock.acquire()
print 'Second try:', lock.acquire(0)

Execution result:

$ python threading_lock_reacquire.py

First try : True
Second try: False

Use RLock to obtain multiple locks:

import threading
lock = threading.RLock()
print 'First try :', lock.acquire()
print 'Second try:', lock.acquire(0)

Execution results:

python threading_rlock.py

First try : True
Second try: 1

Let’s look at another example:

#!/usr/bin/env python3
# coding=utf-8
import threading
import time
class Box(object):
  lock = threading.RLock()
  def __init__(self):
    self.total_items = 0
  def execute(self,n):
    Box.lock.acquire()
    self.total_items += n
    Box.lock.release()
  def add(self):
    Box.lock.acquire()
    self.execute(1)
    Box.lock.release()
  def remove(self):
    Box.lock.acquire()
    self.execute(-1)
    Box.lock.release()
    
## These two functions run n in separate
## threads and call the Box's methods    
def adder(box,items):
  while items > 0:
    print ("adding 1 item in the box\n")
    box.add()
    time.sleep(5)
    items -= 1
    
def remover(box,items):
  while items > 0:
    print ("removing 1 item in the box")
    box.remove()
    time.sleep(5)
    items -= 1
    
## the main program build some
## threads and make sure it works
if __name__ == "__main__":
  items = 5
  print ("putting %s items in the box " % items)
  box = Box()
  t1 = threading.Thread(target=adder,args=(box,items))
  t2 = threading.Thread(target=remover,args=(box,items))
  t1.start()
  t2.start()
  t1.join()
  t2.join()
  print ("%s items still remain in the box " % box.total_items)

Execution result:

$ python3 threading_rlock2.py

##

putting 5 items in the box 
adding 1 item in the box
removing 1 item in the box
adding 1 item in the box
removing 1 item in the box
adding 1 item in the box
removing 1 item in the box
removing 1 item in the box
adding 1 item in the box
removing 1 item in the box
adding 1 item in the box
0 items still remain in the box

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