beginner, thread & else

[Fredrik Lundh]

Gigs_ wrote:

> can someone explain me this code

did you write that yourself, or did you find it in some book or article?

> ----------------------------------
> import thread

the thread module should not be used directly by application programs; 
use the "threading" module instead.

> stdoutmutex = thread.allocate_lock()
> exitmutexes = [0] * 10
> 
> def counter(myId, count):
>      for i in range(count):
>          stdoutmutex.acquire()
>          print '[%s] => %s' % (myId, i)
>          stdoutmutex.release()
>      exitmutexes[myId] = 1  # signal main thread
> 
> for i in range(10):
>      thread.start_new(counter, (i, 100))
> 
> while 0 in exitmutexes:
>      pass

that's a "busy loop"; the CPU will spend all the cycles it can get 
checking for the condition.  that's not a good way to wait for
things.

use the "threading" module instead; by default, it waits for all threads 
to finish without requiring flag lists and excessive CPU use.

> print 'Main thread exiting.'
> -----------------------------------
> 
> thread.start_new(counter, (i, 100)) is running  counter function.

it starts a new instance of counter, in a separate thread.

> Is counter function and while statement executed in same time (other 
> things i understand, but can't get into this)?

yes, the while loop will run in the "main" thread, in parallel with the 
other threads.  however, Python uses a global lock to synchronize access 
to Python variables:

     http://effbot.org/pyfaq/what-is-the-global-interpreter-lock.htm

so the threads in a simple program like this won't run fully in parallel 
on a multi-CPU machine.

</F>