Core Python Programming (2nd Edition)

We did not have any code that told the main thread to wait for the child threads to complete before
continuing. This is what we mean by threads requiring some sort of synchronization. In our case, we
used another sleep() call as our synchronization mechanism. We used a value of 6 seconds because we
know that both threads (which take 4 and 2 seconds, as you know) should have completed by the time
the main thread has counted to 6.
You are probably thinking that there should be a better way of managing threads than creating that
extra delay of 6 seconds in the main thread. Because of this delay, the overall runtime is no better than
in our single-threaded version. Using sleep() for thread synchronization as we did is not reliable. What if
our loops had independent and varying execution times? We may be exiting the main thread too early or
too late. This is where locks come in.
Making yet another update to our code to include locks as well as getting rid of separate loop functions,
we get mtsleep2.py, presented in Example 18.3. Running it, we see that the output is similar to mtsleep1.
py. The only difference is that we did not have to wait the extra time for mtsleep1.py to conclude. By
using locks, we were able to exit as soon as both threads had completed execution.
$ mtsleep2.py
starting at: Sun Aug 13 16:34:41 2006
start loop 0 at: Sun Aug 13 16:34:41 2006
start loop 1 at: Sun Aug 13 16:34:41 2006
loop 1 done at: Sun Aug 13 16:34:43 2006
loop 0 done at: Sun Aug 13 16:34:45 2006
all DONE at: Sun Aug 13 16:34:45 2006
Example 18.3. Using tHRead and Locks (mtsleep2.py)
Rather than using a call to sleep() to hold up the main thread as in mtsleep1.py, the use of
locks makes more sense.
1 #!/usr/bin/env python
2
3 import thread
4 from time import sleep, ctime
5
6 loops = [4,2]
7
8 def loop(nloop, nsec, lock):
9 print 'start loop', nloop, 'at:', ctime()
10 sleep(nsec)
11 print 'loop', nloop, 'done at:', ctime()
12 lock.release()
13
14 def main():
15 print 'starting at:', ctime()
16 locks = []
17 nloops = range(len(loops))
18
19 for i in nloops:
20 lock = thread.allocate_lock()
21 lock.acquire()
22 locks.append(lock)
23
24 for i in nloops: