Software Design
Spring 2007

Processes
---------

A process is a running program.  It contains:

1) a copy of the program code
   (the Python interpreter plus the Python program)

2) program data (global variables, but not local variables)

3) initially, one thread of execution, which contains

   a) a program counter (where in the program are we?)

   b) a stack of frames (which contain local variables)


Threads
-------

Abstractly, a thread represents the flow of execution in a program.

Usually, a program is executed in a single thread, meaning that
at any given time, only one thing is happening.

But many interactive programs run multiple threads.

For example, at the same time a web browser might

1) download a file

2) render an image

3) play back a sound file

4) update an animated GIF

5) handle events from the user

Each of these activities would happen in a separate thread.

At a point in time, different threads are executing different
parts of the program.


Python Threads
--------------

Download thread.py from the usual place and run it:

from threading import Thread
from time import sleep

def counter(xs, delay=1):
    for x in xs:
        print x
        sleep(delay)

# one thread counts backwards, fast
t = Thread(target=counter, args=[range(100, 1, -1), 0.25])
t.start()

# the other thread counts forward, slowly
counter(range(1, 100), 1)


What happens if you use Ctrl-C to try to stop?


Q) How can you stop a running thread?

A) In general, you can't.  Methods that run in the background
should check periodically to see if they should keep going.

This is a pain.

An alternative is the Watcher class in Gui.py.  Check it out
if you are curious.


Q) Why is the Thread interface so horrible?

A) I don't know, but you can improve it with something like this:

class MyThread(threading.Thread):
    """this is a wrapper for threading.Thread that improves
    the syntax for creating and starting threads.  See Appendix A
    of The Little Book of Semaphores, http://greenteapress.com/semaphores/
    """
    def __init__(self, target, *args):
        threading.Thread.__init__(self, target=target, args=args)
        self.start()

Then you can create and start a thread the same way you make
a Callable:

Thread(counter, range(100, 1, -1), 0.25)