[Python-Dev] New PEP: 319

[Michel Pelletier]

> On Mon, Jun 16, 2003 at 02:19:19AM -0500, Michel Pelletier wrote:
>>     def change(self, delta=1):
>>         synchronize self.counter:
> <             self.counter += delta # typo corrected by jepler
>>
>> No explicit lock is necessary.  Any object may be synchronized upon
>> (except, perhaps, None).  The first time an object is synchronized, a
>> lock
>> is invisibly associated with it behind the scenes, you cannot (and
>> should
>> not) access this lock.  The lock exists for the life of the object it
>> synchronizes.  When a synchronize block is entered, the lock is
>> acquire()d
>> and and release()d when the block is exited.
>
> I don't see how this can possibly work.  It looks like self.counter is
> an int, so the statement
>     synchronize self.counter: ...
> must be using some particular int (say, 3) for purposes of
> synchronization.  What sense does this make?

Hmm good point, integer objects are a special case, they  are shared and
are thus a bad example.  Perhaps only instances should be synchronizable.

>  (and where can you store
> the lock, since an int is immutable and can't have new attributes
> created?)

that's up to the implementation.  Lock association does not effect
mutability.

> On the other hand, if the thing you're locking is the counter
> attribute of slot (and ignoring for a moment where the lock is stored)
> then what if self.counter is a list but id(self.counter) ==
> id(globallist)?

If they have the same id() they are the same object and thus the same
associated lock. the below code will be prevented from executing at the
same time.

>  Then the 'synchronize' will not prevent these two
> snippets from executing at the same time:
>     def change(self, delta=1):
>         synchronize self.counter:
>             for i in range(delta):
>                 self.counter.append(i)
>
>     synchronize globallist:
>         globallist.pop()
> globallist.pop() could now see a different item than delta-1
>
> My other question concerns the 'asynchronize' portion of your proposal.
> Is this from Java, or is it your own innovation?  I didn't turn up
> anything about it in several web searches, but I'm not familiar enough
> with java to know for sure.

Yep that's my idea; but I doubt there's no precedence for it.

-Michel