[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 19:25:03 EDT 2018

On Sun, Jun 24, 2018 at 12:03 PM Tim Peters <tim.peters at gmail.com> wrote:

[Guido] :> However, [Tim] 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)?

Two things to say about that. First, the original example I gave would be approximately as well addressed by allowing to declare intended scopes in magically synthesized functions; like (say) p = None # to establish the intended scope of p while any( # split across lines just for readability n % p == 0 for p in smallprimes): n //= p It didn't really require an inline assignment, just a way to override the unwanted (in this case) "all for targets are local to the invisible function" rigid consequence of the implementation du jour.

Hm, that's more special syntax. The nice bit about (b) as currently specified is that it adds no syntax -- it adds a scope rule, but (as IIRC Steven has convincingly argued) few people care about those. Python's scope rules, when fully specified, are intricate to the point of being arcane (e.g. for class scopes) but all that has a purpose -- to make them so DWIM ("Do what I Mean") that in practice you almost never have to worry about them, especially when reading non-obfuscated code (and also when writing, except for a few well-known patterns).

Second, if it's dropped, then the PEP needs more words to define what happens in cases like the following, because different textual parts of a comprehension execute in different scopes, and that can become visible when bindings can be embedded:

def f(): y = -1 ys = [y for in range(y := 5)] print(y, ys) Here range(y := 5) is executed in f's scope. Presumably the y in y_ _for also refers to f's scope, despite that y textually appears to be assigned to in the body of the listcomp, and so would - for that reason - expected to be local to the synthesized function, and so raise UnboundLocalError when referenced. It's incoherent without detailed knowledge of the implementation.

That code should have the same meaning regardless of whether we accept (b) or not -- there is only one y, in f's scope. I don't mind if we have to add more words to the PEP's scope rules to make this explicit, though I doubt it -- the existing weirdness (in the comprehension spec) about the "outermost iterable" being evaluated in the surrounding scope specifies this. I wouldn't call it incoherent -- I think what I said about scope rules above applies here, it just does what you expect.

def g(): y = -1 ys = [y for y in range(y := 5)] print(y, ys)

And here the y in y for y is local to the synthesized function, and presumably has nothing to do with the y in the range() call. That's incoherent in its own way. Under the current PEP, all instances of y in f refer to the f-local y, and the listcomp in g is a compile-time error.

And under the (b)-less proposal, g would interpret y for y as both referring to a new variable created just for the comprehension, and y := 5 as referring to g's scope. Again I don't think it needs extra words in the spec. And the end user docs might just say "don't do that" (with a link to the reference manual's rule about the "outermost iterable").

Even if in the end we did find a case where we'd have to write an explicit rule to make what happens here a consequence of the spec rather than the implementation, that doesn't count as an argument for keeping (b) to me.

In favor of (b) we have a few examples (see https://www.python.org/dev/peps/pep-0572/#scope-of-the-target) that require it, and more that you described on python-ideas (and also the motivating use case from the thread that Steven dug up, starting here: https://mail.python.org/pipermail/python-ideas/2018-April/049622.html).

A "neutral" argument about (b) is that despite the "horrified" reactions that Nick saw, in practice it's going to confuse very few people (again, due to my point about Python's scope rules). I'd wager that the people who might be most horrified about it would be people who feel strongly that the change to the comprehension scope rules in Python 3 is a big improvement, and who are familiar with the difference in implementation of comprehensions (though not generator expressions) in Python 2 vs. 3.

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