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

Nathaniel Smith njs at pobox.com
Sat May 25 17:09:20 EDT 2019

On Sat, May 25, 2019, 07:38 Guido van Rossum <guido at python.org> wrote:

This looks great.

I only have two nits with the text. First, why is the snapshot called a "dynamic snapshot"? What exactly is dynamic about it?

It's dynamic in that it can spontaneously change when certain other events happen. For example, imagine this code runs at function scope:

take a snapshot

a = locals()

it's a snapshot, so it doesn't include the new variable

assert "a" not in a

take another snapshot

b = locals()

now our first "snapshot" has changed

assert "a" in a

Overall I'm happy with the PEP, but I'm still a bit uneasy about whether we've gotten the details of this "dynamicity" exactly right, esp. since the PEP promotes them from implementation detail to language features. There are a lot of complicated tradeoffs so I'm working on a longer response that tries to lay out all the options and hopefully convince myself (and everyone else).

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