[Tutor] "Lock"ing threads

[Hans Dushanthakumar]

Hi,
   In a multithreaded program, how do I ensure that a block of code in a
thread is always executed fully before passing control to another
thread. Does "Lock" serve this purpose?

The foll program is a dummy one, with 2 threads. One put a number onto a
queue (of max size 1) and the other thread reads from the que.

However, on running this program (Win XP, NOT via IDLE - it hangs when I
run it thru IDLE) the output that I see on screen indicates that the
block of code within the lock aquire and release was not run completely
before the other thread started running. Note that the print messages
from the 2 threads seem to be interspersed together:


import threading
import Queue

class put_num(threading.Thread):
    stop_thread = 0

    def __init__(self, num, que):
        threading.Thread.__init__(self)
        self.que = que
        self.num = num
        self.lock = threading.Lock()
        
    def run(self):
        global stop_thread
        for k in range (20):
            self.lock.acquire()
            print "put_num: ", self.num
            self.que.put(str(self.num))
            print "put_num: Que size = ", self.que.qsize()
            self.num = self.num + 1
            self.lock.release()

class get_num(threading.Thread):
    stop_thread = 0

    def __init__(self, que):
        threading.Thread.__init__(self)
        self.que = que
        self.lock = threading.Lock()
        
    def run(self):
        global stop_thread
        for k in range (20):
            self.lock.acquire()
            mynum = self.que.get()
            print "get_num: ", mynum
            print "get_num: Que size = ", self.que.qsize()
            self.lock.release()

my_que = Queue.Queue(1)

put_num_thread = put_num(742, my_que)
get_num_thread = get_num(my_que)

print "Starting threads"
put_num_thread.start()
get_num_thread.start()

print "Waiting for threads to finish"
put_num_thread.join()
get_num_thread.join()

print "Closing down"
raw_input("\n\nPress enter to Quit: ")




This is the out put of the above program:



Starting threads
put_num:  742
Waiting for threads to finish
put_num: Que size =  1
get_num:  742
get_num: Que size =  0
put_num:  743
put_num: Que size =  1
get_num:  743
get_num: Que size =  0
put_num:  744
put_num: Que size =  1
get_num:  744
get_num: Que size =  0
put_num:  745
put_num: Que size =  1
get_num:  745
get_num: Que size =  0
put_num:  746
put_num: Que size =  1
get_num:  746
get_num: Que size =  0
put_num:  747
put_num: Que size =  get_num:  747
get_num: Que size =  0
0
put_num:  748
put_num: Que size =  1
get_num:  748
get_num: Que size =  0
put_num:  749
put_num: Que size =  get_num:  749
get_num: Que size =  0
0
put_num:  750
put_num: Que size =  1
get_num:  750
get_num: Que size =  0
put_num:  751
put_num: Que size =  1
get_num:  751
get_num: Que size =  0
put_num:  752
put_num: Que size =  get_num:  752
get_num: Que size =  0
0
put_num:  753
put_num: Que size =  1
get_num:  753
get_num: Que size =  0
put_num:  754
put_num: Que size =  1
get_num:  754
get_num: Que size =  0
put_num:  755
put_num: Que size =  get_num:  755
get_num: Que size =  0
0
put_num:  756
put_num: Que size =  get_num:  756
get_num: Que size =  0
0
put_num:  757
put_num: Que size =  get_num:  757
get_num: Que size =  0
0
put_num:  758
put_num: Que size =  1
get_num:  758
get_num: Que size =  0
put_num:  759
put_num: Que size =  get_num:  759
get_num: Que size =  0
0
put_num:  760
put_num: Que size =  1
get_num:  760
get_num: Que size =  0
put_num:  761
put_num: Que size =  get_num:  761
get_num: Que size =  0
0
Closing down


Press enter to Quit: