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

On Tue, May 28, 2019 at 5:25 PM Greg Ewing <greg.ewing at>

> Terry Reedy wrote:
> > 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.
> Yes, I understand *what's* happening, but not *why* it was designed
> that way. Would it really be prohibitively expensive to create a
> fresh dict each time?

No. But it would be inconsistent with the behavior at module level.

FWIW I am leaning more and more to the [proxy] model, where locals() and
frame.f_locals are the same object, which *proxies* the fast locals and
cells. That only has one downside: it no longer returns a dict, but merely
a MutableMapping. But why would code care about the difference? (There used
to be some relevant builtins that took dicts but not general
MutableMappings -- but that has been fixed long ago.)

--Guido van Rossum (
*Pronouns: he/him/his **(why is my pronoun here?)*
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