Python threading tutorial?

hg hansgeunsmeyer at earthlink.net
Sun Jan 5 19:43:19 CET 2003


Gerrit Holl <gerrit at nl.linux.org> wrote in message 

> ...so I am currently not using the queues. Does that mean my application
> is not reliable, or is this a reliable solution? It seems to run ok...

It's definitely not reliably with your Matrix class, and I'm surprised
you haven't noticed any difficulties yet.

> 
> My source code:

> class Person(threading.Thread):
...
> 	def move(self, dir):
                ...
> 		if isinstance(self.mx[newx][newy], self.__class__):
> 			raise Collision
> 
> 		self.mx[self.x][self.y] = self.mx.bg
> 		self.mx[newx][newy] = self
> 		self.x = newx
> 		self.y = newy
> ...

For instance, if there is a thread switch immediately after isinstance
the state of your "world" may become inconsistent:

- thread A: person A sees that (newx, newy) is empty
- thread-switch to B: person B sees that (newx, newy) is empty, and
occupies those coordinates
- thread-switch back to A: person A occupies the same coordinates...

So, in class Matrix you have to add some kind of locking (see
Queue.py). Or you have to add some locking mechanism to function move.

Hans




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