On Thu, Oct 13, 2016 at 11:32:49PM -0400, Random832 wrote: > On Thu, Oct 13, 2016, at 18:15, Steven D'Aprano wrote: > > Consider the analogy with f(*t), where t = (a, b, c). We *don't* have: > > > > f(*t) is equivalent to f(a); f(b); f(c) > > I don't know where this "analogy" is coming from.

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I'm explicitly saying that we DON'T have that behaviour with function calls. f(*t) is NOT expanded to f(a), f(b), f(c). I even emphasised the "don't" part of my sentence above. And yet, this proposal wants to expand [*t for t in iterable] into the equivalent of: result = [] for t in iterable: a, b, c = *t result.append(a) result.append(b) result.append(c) Three separate calls to append, analogous to three separate calls to f(). The point I am making is that this proposed change is *not* analogous to the way sequence unpacking works in other contexts. I'm sorry if I wasn't clear enough. [...] > > Indeed. The reader may be forgiven for thinking that this is yet another > > unrelated and arbitrary use of * to join the many other uses: > > How is it arbitrary? It is arbitrary because the suggested use of *t in list comprehensions has no analogy to the use of *t in other contexts. As far as I can see, this is not equivalent to the way sequence (un)packing works on *either* side of assignment. It's not equivalent to the way sequence unpacking works in function calls, or in list displays. It's this magical syntax which turns a virtual append() into extend(): # [t for t in iterable] result = [] for t in iterable: result.append(t) # but [*t for t in iterable] result = [] for t in iterable: result.extend(t) or, if you prefer, keep the append but magical add an extra for-loop: # [*t for t in iterable] result = [] for t in iterable: for x in t: result.append(x) > > - mathematical operator; > > - glob and regex wild-card; > > - unpacking; > > This is unpacking. It unpacks the results into the destination. If it were unpacking as it is understood today, with no other changes, it would be a no-op. (To be technical, it would convert whatever iterable t is into a tuple.) I've covered that in an earlier post: if you replace *t with the actual items of t, you DON'T get: result = [] for t in iterable: a, b, c = *t # assuming t has three items, as per above result.append(a) result.append(b) result.append(c) as desired, but: result = [] for t in iterable: a, b, c = *t result.append((a, b, c)) which might as well be a no-op. To make this work, the "unpacking operator" needs to do more than just unpack. It has to either change append into extend, or equivalently, add an extra for loop into the list comprehension. > There's a straight line from [*t, *u, *v] to [*x for x in (t, u, v)]. > What's surprising is that it doesn't work now. I'm not surprised that it doesn't work. I expected that it wouldn't work. When I first saw the suggestion, I thought "That can't possibly be meaningful, it should be an error." Honestly Random832, I cannot comprehend how you see this as a straightforward obvious extension from existing behaviour. To me, this is nothing like the existing behaviour, and it contradicts the way sequence unpacking works everywhere else. I do not understand the reasoning you use to conclude that this is a straight-line extension to the current behaviour. Nothing I have seen in any of this discussion justifies that claim to me. I don't know what you are seeing that I cannot see. My opinion is that you're seeing things that aren't there -- I expect that your opinion is that I'm blind. > I think last month we even had someone who didn't know about 'yield > from' propose 'yield *x' for exactly this feature. It is intuitive - it > is a straight-line extension of the unpacking syntax. Except for all the folks who have repeatedly said that it is counter-intuitive, that it is a twisty, unexpected, confusing path from the existing behaviour to this proposal. -- Steve _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/