Question about expression evaluation

Peter Otten __peter__ at
Mon Nov 8 19:07:23 CET 2010

Scott Gould wrote:

> Thanks for the ideas, everyone. It's probably obvious that this is in
> a Django context, and while I do have WSGI configured to multi-thread
> its processes, there is nothing explicitly shared -- via threading, a
> multi-user situation, or otherwise -- about this data. It is entirely
> local to the request so I can't figure out how the "contact" object
> could change in a single line of code.
> But obviously there must be something going on like what you all
> suggest. I just can't figure out how. This is the entirety of the
> contact-producing code within Account:
>         @property
>         def contact(self):
>                 if not hasattr(self, '_contact'):

If one thread stops at this point the _contact attribute is not yet set and 
another thread may trigger the if-suite, too.

>                         self._contact = 
get_contact_for_id( if self.profile
> and else None
>                 return self._contact
> As you can see, it's essentially read-only so once a self._contact
> always a self._contact. (Unless I'm misunderstanding something
> fundamental, which as an admitted amateur I could well be!)

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