[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part) (original) (raw)

Guido van Rossum guido at python.org
Sun Jun 24 19:02:17 EDT 2018

On Sun, Jun 24, 2018 at 11:50 AM Steven D'Aprano <steve at pearwood.info> wrote:

[Guido] > [...] IIRC (b) originated with Tim.

I'm not sure who came up with the idea first, but as I remember it, the first mention of this came in a separate thread on Python-Ideas: https://mail.python.org/pipermail/python-ideas/2018-April/049631.html so possibly I'm to blame :-)

Actually that post sounds like the OP of that thread (Peter O'Connor) is to blame -- he proposed a similar thing using = for the assignment and custom syntax (from <value>) to specify the initial value, and it looks like that inspired you.

> But his essay on the topic, included as Appendix A ( > https://www.python.org/dev/peps/pep-0572/#appendix-a-tim-peters-s-findings ) > does not even mention comprehensions. However, he did post his motivation > for (b) on python-ideas, IIRC a bit before PyCon; and the main text of the > PEP gives a strong motivation ( > https://www.python.org/dev/peps/pep-0572/#scope-of-the-target). > Nevertheless, maybe we should compromise and drop (b)?

I will have more to say about the whole "comprehensions are their own scope" issue later. But I'd like to see Nick's proposed PEP, or at least a draft of it, before making any final decisions.

Agreed, though I assume it's just given again.

If it came down to it, I'd be happy with the ability to declare an assignment target nonlocal in the comprehension if that's what it takes. What do you think of this syntax?

[global|nonlocal] simpletarget := expression Inside a comprehension, without a declaration, the target would be sublocal (comprehension scope); that should make Nick happier :-)

It's more special syntax. Just taking part (a) of PEP 572 would make most people happy enough.

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