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

Steven D'Aprano steve at pearwood.info
Sun May 26 06:04:11 EDT 2019

Richard, your email seems to have introduced a spurious "SPAM" label to this thread, which may confuse some email clients into treating it as spam. Can you teach your email program that this mailing list is ham, not spam, or failing that, at least edit the subject line to remove the label? Thanks. I've done so for this response, but please take care that you don't re-introduce the label again, thanks.

On Sat, May 25, 2019 at 06:37:22PM -0400, Richard Damon wrote:

To me that is a static snapshot of a dynamic environment, not a dynamic snapshot. The snapshot you get at THAT moment in time won't change, as time progresses, so that snapshot itself isn't dynamic.

Actually, it does change -- but the confusing part is that it doesn't change automatically but only when you call the locals() function again. This already CPython's behaviour, so that is not changing.

def demo1(): a = b = c = 1 a = locals() print(a) b = 999 print(a)

def demo2(): a = b = c = 1 a = locals() print(a) b = 999 locals() # call function but throw the result away print(a)

And running those two functions in Python 3.5:

py> demo1() # No change to the dict. {'a': 1, 'b': 1, 'c': 1} {'a': 1, 'b': 1, 'c': 1}

py> demo2() # Dict magically updates! {'a': 1, 'b': 1, 'c': 1} {'a': {...}, 'b': 999, 'c': 1}

I know this is the backwards-compatible behaviour, but I would like to question whether we want to enshrine it in stone. This seems to me to be the worst possible combinations of features:

If this wasn't already the behaviour, would we want it?

-- Steven

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