[Python-ideas] Fwd: quantifications, and tuple patterns (original) (raw)

Guido van Rossum guido at python.org
Sun Jan 15 00:07:49 CET 2012

On Sat, Jan 14, 2012 at 1:38 PM, Paul Moore <p.f.moore at gmail.com> wrote:

On 14 January 2012 19:24, Guido van Rossum <guido at python.org> wrote: > But Paul, aren't you missing the fact that for the algorithms that Annie and > her students want to write, the "witness" concept is essential? I.e. they > can't just use any(P(x) for x in xs) because if it returns True, they want > to know the x that made P(x) be true. Her ! notation is a (perhaps > unpythonic) attempt at exporting this witness from the quantification.

Fair point. I was thinking that common cases worked with any(), and more complex cases that needed a witness would be sufficiently rare that the extra verbosity would (a) not be a huge burden, and (b) help to make the intent clearer. >> > while some (!slotnum,p1) in decisions: >> > if some (!slotnum,p2) in proposals has p2 != p1: >> > propose(p2) >> > perform(p1) [...] >> Can I suggest you write your example out in Python that works today, >> and then show how it looks with your proposed syntax alongside? If you >> can't find the "best" way of writing it in existing Python, just write >> it however works, no need to try to make it compact, or elegant. >> There'll be plenty of people here who will show you how to write >> idiomatic Python versions of what you post :-) > > Actually she gave one in her first post. Here it is again: I'm sorry about that! I got confused part-way through the original post and skimmed from there, and then didn't go back and reread it in the context of the follow-up, so I missed the example. My mistake. > while {p1 for (s0,p1) in decisions if s0==slotnum}: > p1 = {p1 for (s0,p1) in decisions if s0==slotnum}.pop() > for p2 in {p2 for (s0,p2) in proposals if s0==slotnum if p2 != p1}: > > Note that the set {p1 for (s0,p1) in decisions if s0==slotnum} is computed > twice, once to decide whether to stop, and then again to compute the witness > (p1). Obviously this is inefficient, and that's what she's after. Agreed, that's inefficient, and it also violates DRY - I can easily imagine those two lines getting out of sync after a while...

Actually it's worse. neither expression needs to compute the full set -- they only need to iterate until the first match.

> To make > this same code more efficient in Python, you'd have to do the following, > which is natural for us programmers (since we're so used to working around > limitations and inefficiencies in the systems we work with) but unnatural > for mathematicians, who count to infinity at the drop of a hat:

Hmm, I have an aversion to languages (or constructs) based around theoretical principles. Blame a couple of encounters with Haskell a few years ago. I lost. :-)

In my case the jury is still out, but I'm not worried about Haskell ever overtaking Python. :-)

FWIW, comprehensions also come from these theoretical principles, so it's not all bad...

But it tends to be the over-compressed syntax rather than the ideas that I'm particularly allergic to.

> while True: > temp = {p1 for (s0,p1) in decisions if s0==slotnum} > if not temp: > break > p1 = temp.pop() > > for p2 in {p2 for (s0,p2) in proposals if s0==slotnum if p2 != p1}: > Point taken. On the other hand, if (x := val) was an expression form of assignment, like C's assignment, you could write while temp := {p1 for (s0,p1) in decisions if s0==slotnum}: p1 = temp.pop() for s2 ... which is to my mind as succinct, and clearer to a non-mathematician, as your proposal below (and Annie's proposal). It also builds off a much more commonly requested feature :-) Actually, while any(p1 := p for (s,p) in decisions if s == slotnum): for s2 ... works, and is just as short as your proposal or Annie's.

Yeah, and it also avoids computing the elements of the set beyond the first.

> The 5 tedious lines from "while" through "pop()" would be collapsed into a > single line if you could write > > while some s0, p1 in decisions if s0 == slotnum: > > for p2 in {p2 for (s0,p2) in proposals if s0==slotnum if p2 != p1}: > > > TBH I'm not sure what the !slotnum notation is for -- it appears that > > while some (!slotnum, p1) in decisions: > > is equivalent in Annie's proposal to > > while some s0, p1 in decisions if s0 == slotnum: > > but I'm not sure and it doesn't feel necessary to me.

Yes, I think that's right. It looks like the idea comes from the concept of unification in logic languages (and the ! notation means "don't unify this value, but rather treat it as a fixed value that must match"). Thanks for your analysis, by the way, I think understand the proposal a lot better now.

And thanks for the link with logic languages, that's an area I know even less about...

So, going back to what Annie was referring to, there seem to be three key concepts:

Quantifications, which are covered in Python by any() and all() Capturing a "witness", which can be done using assignment-as-expression Tuple matching, which you have shown can be handled using tuple unpacking plus the generator expression if clause, but could probably gain from a more compact notation.

I'm not sure we need a new construct for tuple matching. Witness capturing seems the most important missing feature here.

BTW, as I'm sure everyone knows, you can simulate assignment-as-expression with

def asgn(o, val): o.ans = val return val >>> any(asgn(c,x) for x in (1,2,3) if x%2 == 0) True >>> c.ans 2

Eew. :-(

> Also note that SOME and EACH quantifiers were present in ABC (Python's > predecessor: http://homepages.cwi.nl/~steven/abc/qr.html#TESTS); I dropped > them for simplicity, not because I didn't like them. If we wanted to we > could have them back (except for the problems of introducing new keywords).

One day, I really must read up on ABC.

Just follow the above link and scroll to the top. :-)

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