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

Guido van Rossum guido at python.org
Tue May 28 21:02:19 EDT 2019

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

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 (python.org/~guido) *Pronouns: he/him/his **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20190528/d1775d35/attachment.html>

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