CODEWITHSUNDEEP
pythonmultithreading
Shades88
Shades88

Reputation: 8360

locking problems in a threading example in python

I am just playing around with threads in python and am quite new to python too. I have a producer class of whom I create threads. These thread all access a single object that is a common resource. Below is the code

class Producer (threading.Thread):
    def __init__(self, threadId, source):
    threading.Thread.__init__(self)
    self.source = source
    self.threadId = threadId

    def produce(self):
    while 1:
        data = self.source.getData()
        if data == False:
            print "===== Data finished for "+self.threadId+" ====="
            break
        else:
            print data

    def run(self):
        self.produce()
#class

class A:
    def __init__(self):
        self.dataLimit = 5
        self.dataStart = 1

    def getData(self):
        lock = Lock()
        lock.acquire()
        if self.dataStart > self.dataLimit:
           return False
        lock.release()

        data = "data from A :: "+str(self.dataStart)+" Accessor thread :: "+thread.threadId
        time.sleep(0.5)

        lock.acquire()
        self.dataStart += 1
        lock.release()

        return data
   #def
#class

source = A()
for i in range(2):
    thread = Producer( "t_producer"+str(i), source )
    thread.start()


print "Main thread exiting..."

So class A counts dataStart from 1 to 5. Now as it is a common resource and getData method implements locking too, threads of producer class will access getData method interchangeably and expected output is as follows:

data from A :: 1 Accessor thread :: t_producer0
data from A :: 2 Accessor thread :: t_producer1
data from A :: 3 Accessor thread :: t_producer1
data from A :: 4 Accessor thread :: t_producer0
data from A :: 5 Accessor thread :: t_producer0
===== Data finished for t_producer0 =====
===== Data finished for t_producer1 =====

But I am getting this:

data from A :: 1 Accessor thread :: t_producer0
data from A :: 1 Accessor thread :: t_producer1
data from A :: 3 Accessor thread :: t_producer1
data from A :: 3 Accessor thread :: t_producer1
data from A :: 5 Accessor thread :: t_producer1
===== Data finished for t_producer0 =====
data from A :: 5 Accessor thread :: t_producer1
===== Data finished for t_producer1 =====

As you can see data count is repeating, randomly count is missing. How to deal with this problem here?

Upvotes: 1

Views: 1793

Answers (2)

danny
danny

Reputation: 1

def getData(self):
        lock = threading.Lock()
        with lock:
            if self.dataStart > self.dataLimit:
                return False

            data = "data from A :: " + str(self.dataStart) + " Accessor thread :: " + thread.threadId
            self.dataStart += 1
        return data

Upvotes: -1

Wessie
Wessie

Reputation: 3520

def getData(self):
    lock = Lock()
    lock.acquire()
    if self.dataStart > self.dataLimit:
       return False
    lock.release()

    data = "data from A :: "+str(self.dataStart)+" Accessor thread :: "+thread.threadId
    time.sleep(0.5)

You are returning False before the release call. Try using the with statement like so:

with lock:
    # Do stuff

This will make sure to acquire and then release it afterwards.

Upvotes: 4

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