I don't understand what is happening in this threading code

[Matthew Wilson]

In this code, I tried to kill my thread object by setting a variable on it
to False.

Inside the run method of my thread object, it checks a different
variable.

I've already rewritten this code to use semaphores, but I'm just curious
what is going on.

Here's the code:

import logging, threading, time
logging.basicCo nfig(level=logg ing.DEBUG,
format="%(threa dName)s: %(message)s")

class Waiter(threadin g.Thread):
def __init__(self, hot_food):
super(Waiter, self).__init__( )
self.should_kee p_running = True
self.hot_food = hot_food

def run(self):
while self.should_kee p_running:
logging.debug(" Inside run, the id of should_keep_run ning is %s."
% id(self.should_ keep_running))
self.hot_food.a cquire()

def cook_food(hot_f ood):
i = 5
while i >= 0:
logging.debug(" I am cooking food...")
time.sleep(1)
hot_food.releas e()
logging.debug(" Andiamo!")
i -= 1

def main():

hot_food = threading.Semap hore(value=0)

chef = threading.Threa d(name="chef", target=cook_foo d, args=(hot_food, ))
chef.start()

w = Waiter(hot_food )
logging.debug(" Initially, the id of w.should_keep_r unning is %s."
% id(w.should_kee p_running))
w.start()
logging.debug(" After start, the id of w.should_keep_r unning is %s."
% id(w.should_kee p_running))

# Wait for the chef to finish work.
chef.join()

# Now try to kill off the waiter by setting a variable inside the waiter.
w.should_keep_r unning = False
logging.debug(" Now, the id of w.should_keep_r unning is %s."
% id(w.should_kee p_running))

if __name__ == "__main__":
main()

And here's what I get when I execute it. I have to suspend the process
with CTRL=Z and then kill -9 it.

$ python foo.py
MainThread: Initially, the id of w.should_keep_r unning is 135527852.
MainThread: After start, the id of w.should_keep_r unning is 135527852.
chef: I am cooking food...
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
chef: Andiamo!
chef: I am cooking food...
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
chef: Andiamo!
chef: I am cooking food...
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
chef: Andiamo!
chef: I am cooking food...
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
chef: Andiamo!
chef: I am cooking food...
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
chef: Andiamo!
chef: I am cooking food...
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
chef: Andiamo!
Thread-1: Inside run, the id of should_keep_run ning is 135527852.
MainThread: Now, the id of w.should_keep_r unning is 135527840.

[1]+ Stopped python foo.py

$ kill -9 %1

[1]+ Stopped python foo.py

The memory address of should_keep_run ning seems to change when I set it
from True to False, and inside the run method, I keep checking the old
location.

I am totally baffled what this means.

Like I said earlier, I already rewrote this code to use semaphores, but
I just want to know what is going on here.

Any explanation is welcome.

TIA

Matt

[Carl Banks]

On Jan 18, 7:43 pm, Matthew Wilson <[e-mail] mwrote:

In this code, I tried to kill my thread object by setting a variable on it
to False.

Inside the run method of my thread object, it checks a different
variable.

I've already rewritten this code to use semaphores, but I'm just curious
what is going on.

Here's the code:

import logging, threading, time
logging.basicCo nfig(level=logg ing.DEBUG,
format="%(threa dName)s: %(message)s")

class Waiter(threadin g.Thread):
def __init__(self, hot_food):
super(Waiter, self).__init__( )
self.should_kee p_running = True
self.hot_food = hot_food

def run(self):
while self.should_kee p_running:
logging.debug(" Inside run, the id of should_keep_run ning is %s."
% id(self.should_ keep_running))
self.hot_food.a cquire()

def cook_food(hot_f ood):
i = 5
while i >= 0:
logging.debug(" I am cooking food...")
time.sleep(1)
hot_food.releas e()
logging.debug(" Andiamo!")
i -= 1

def main():

hot_food = threading.Semap hore(value=0)

chef = threading.Threa d(name="chef", target=cook_foo d, args=(hot_food, ))
chef.start()

w = Waiter(hot_food )
logging.debug(" Initially, the id of w.should_keep_r unning is %s."
% id(w.should_kee p_running))
w.start()
logging.debug(" After start, the id of w.should_keep_r unning is %s."
% id(w.should_kee p_running))

# Wait for the chef to finish work.
chef.join()

# Now try to kill off the waiter by setting a variable inside the waiter.
w.should_keep_r unning = False
logging.debug(" Now, the id of w.should_keep_r unning is %s."
% id(w.should_kee p_running))

if __name__ == "__main__":
main()

It looks like your program's hanging while the waiter is waits to
acquire another plate of hot food.

Quick and dirty solution is to have waiter timeout at intervals while
waiting for the chef and check the clock to see if his shift has
ended. Better solution is to rewrite the logic, which you did.
Carl Banks

[Tim Roberts]

Matthew Wilson <ma**@tplus1.co mwrote:

>In this code, I tried to kill my thread object by setting a variable on it
to False.

Inside the run method of my thread object, it checks a different
variable.

I've already rewritten this code to use semaphores, but I'm just curious
what is going on.
...
The memory address of should_keep_run ning seems to change when I set it
from True to False, and inside the run method, I keep checking the old
location.

You misunderstand what "id(a)" does. "id" doesn't tell you anything about
the name "a" or the address of "a". What it tells you is the unique
identifier of the object that "a" is bound to. Any time you change "a",
"id(a)" will also change.

Note, for example:

C:\tmp>python
Python 2.4.4 (#71, Oct 18 2006, 08:34:43) [MSC v.1310 32 bit (Intel)] on
win32
Type "help", "copyright" , "credits" or "license" for more information.

>>id(True)

504958236

>>a = True
id(a)

504958236

>>id(False)

504958224

>>a = False
b = False
id(a)

504958224

>>id(b)

504958224

>>>

504958236 is the id of "True", not of "a".
--
Tim Roberts, ti**@probo.com
Providenza & Boekelheide, Inc.