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[Python-Dev] PEP 558: Defined semantics for locals()


No, there's only one locals() dict *per stack frame*. So no worries about
concurrency.

On Mon, May 27, 2019 at 6:54 AM Richard Damon <Richard at damon-family.org>
wrote:

> On 5/27/19 9:12 AM, Terry Reedy wrote:
> > On 5/27/2019 3:18 AM, Greg Ewing wrote:
> >> Chris Angelico wrote:
> >>> Except that it does. After calling locals() a second time, the result
> >>> of the *first* call will be updated to reflect changes.
> >>
> >> Yeow. That's *really* unintuitive. There had better be an extremely
> >> good reason for this behaviour.
> >
> > I believe that the situation is or can be thought of as this: there is
> > exactly 1 function locals dict.  Initially, it is empty and
> > inaccessible (unusable) from code.  Each locals() call updates the
> > dict to a current snapshot and returns it.
> >
> I had a similar concern, and one BIG issue with it being define this way
> is that you get a fundamental re-entrancy problem. If module a uses
> locals(), and then calls module b that uses locals(), module a has lost
> its usage. One implication of this is that then you really want ALL
> modules to define if they use the locals() function or not, then you get
> the question, does this 1 of apply across threads? does a call to locals
> in another thread make me lose my locals (or does each thread get its
> own version), if that is true then if you might possible be in a
> situation where threads are in play you MUST make the copy anyway, and
> do it fast enough that the GIL isn't released between the snapshot and
> the copy (if possible),
>
> C made this sort of mistake decades ago for some functions, not thinking
> about threads or re-entrancy, and had to create solutions to fix it. Let
> us not forget history and thus repeat it.
>
> Is there a fundamental reason that local needs to keep a single dict, as
> opposed to creating a new one for each call? The way it is currently
> defined, once it is called, the snapshot will stay forever, consuming
> resources, while if a new dict was created, the resource would be
> reclaimed after use. Yes, if called twice you end up with two copies
> instead of both being updated to the current, but if you WANTED to just
> update the current copy, you could just rebind it to the new version,
> otherwise you are just forcing the programmer to be making the copies
> explicitly.
>
> --
> Richard Damon
>
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