Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Steven D'Aprano pearwood.info> writes: > > If you can think of any other way to efficiently cycle over the elements > in a set, I'm all for it :) How about "for x in s"? Or if you want to cycle: >>> s = set('abc') >>> it = itertools.cycle(s) >>> next(it) 'a' >>> next(it) 'c' >>> next(it) 'b'

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

[Stephen Turbull] > The resources available for language evolution have dried up; > *you* who want more language evolution are going to have to > supply them. That's the idea I've been advocating- a code-first-jaw-later approach and the sandbox to support it. Its not even that hard to find where I

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

geremy condra writes: > On Wed, Oct 28, 2009 at 12:34 PM, average wrote: > > [I wrote:] > >> If Python3 were to have this feature it would make it worth > >> migrating to > > > > Sorry that may have sounded more harsh than I expected.  If I had more > > resources, > This is effectively th

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Wed, Oct 28, 2009 at 12:34 PM, average wrote: > [I wrote:] >> If Python3 were to have this feature it would make it worth >> migrating to > > Sorry that may have sounded more harsh than I expected.  If I had more > resources, I'd propose (and volunteer) a python3000 branch where any > and all w

[Python-Dev] Retrieve an arbitrary element from a set without removing it

[I wrote:] > If Python3 were to have this feature it would make it worth > migrating to Sorry that may have sounded more harsh than I expected.  If I had more resources, I'd propose (and volunteer) a python3000 branch where any and all who were disappointed at the *lack* of compatability changes c

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

[Guido wrote:] > - If sets were to grow an API to non-destructively access the object > stored in the set for a particular key, then dicts should have such a > method too. > - I still wish we could go back in time and unify sets and dicts, if > only to find out how that experiment would turn out.

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Alexander Belopolsky wrote: > Odd indeed. My first reaction was: it is not needed because lists > support slicing, but when I tried to construct a list.get() using > slicing the best I could come up with was the following hack > def lget(l, i, v): return (l[i:] or [v])[0] > ... lget(ra

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Tue, Oct 27, 2009 at 1:33 PM, Chris Bergstresser wrote: > On Tue, Oct 27, 2009 at 11:06 AM, Georg Brandl wrote: >> Sorry to nitpick, but there is no list.get(). > >   No?  How ... odd. Odd indeed. My first reaction was: it is not needed because lists support slicing, but when I tried to cons

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Chris Bergstresser schrieb: > On Tue, Oct 27, 2009 at 11:06 AM, Georg Brandl wrote: >> Sorry to nitpick, but there is no list.get(). > >No? How ... odd. I guess it wouldn't have come up, but I was sure > there was a .get method which took an optional default parameter if > the index didn't

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Tue, Oct 27, 2009 at 11:06 AM, Georg Brandl wrote: > Sorry to nitpick, but there is no list.get(). No? How ... odd. I guess it wouldn't have come up, but I was sure there was a .get method which took an optional default parameter if the index didn't exist, mirroring the dict method. Stil

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Chris Bergstresser schrieb: >I like the proposed set.get() method, personally. list.get(index) > gets the item at that index, dict.get(key) gets the item associated > with that key, set.get() gets an item, but doesn't place any > guarantees on which item is returned. Sorry to nitpick, but th

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Mon, Oct 26, 2009 at 11:38 AM, Guido van Rossum wrote: > - If sets were to grow an API to non-destructively access the object > stored in the set for a particular key, then dicts should have such a > method too. > > - Ditto for an API to non-destructively get an arbitrary element. > > - I'm far

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

All this talk of equivalence classes makes me dizzy. :-) - If sets were to grow an API to non-destructively access the object stored in the set for a particular key, then dicts should have such a method too. - Ditto for an API to non-destructively get an arbitrary element. - I'm far from convinc

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Alexander Belopolsky wrote: Here is an alternative idea on how storing interned objects in a set can be supported. Currently set.add method returns None and had no effect when set already has an object equal to the one being added. I propose to consider changing that behavior to make set.add r

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sun, Oct 25, 2009 at 1:48 AM, Nick Coghlan wrote: > Terry Reedy wrote: >>> In this model, a >>> get() method, or even a __getitem__ with s[k] is k, is only natural. >> >> No, if key and value were the same thing, the get method would be >> nonesense, as Guido said. But since they are not, sinc

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Hi, I totally agree regarding the efficiency. Code that relies on a fast "non- removing .pop()" probably has other worse bottlenecks that should be targetted first. This would, however, relief every programmer who needs this the first time in his Python experience, to research how this could be

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

> Why not allow that? > > def any(self, predicate=lambda x: True, default=None) > for a in self: > if predicate(a): > break > else: > return default > return a I'm +0 (given that I'm still skeptical about the need to have somethi

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

"Martin v. Löwis" writes: > > res.get() would be a fairly obvious way to do it. Enough that I would > > probably never have gone searching for any of the other answers. Though > > personally, I think I would call it "set.peek()", but the specific name > > doesn't really matter to me. > > So

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

[John Arbash Meine] So 'for x in s: break' is about 2x faster than next(iter(s)) and 3x faster than (iter(s).next()). The first version uses direct calls for the for-loop opcodes. The other two have to do functions/method look-up and make a pure python function call (neither is very fast). FW

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

John Arbash Meinel wrote: res = heads(node1, node2) if len(res) == 1: # What is the 'obvious' way to get the node out? I posit that there *isn't* an obvious way to get the single item out of a 1-entry frozenset. for x in res: break list(res)[0] set(res).pop() iter(res).next() [x for x in res]

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

> We were using sets to track the tips of a graph, and to compare whether > one node was an ancestor of another one. We were caching that answer > into frozensets, since that made them immutable. If > > res = heads(node1, node2) > if len(res) == 1: > # What is the 'obvious' way to get the node o

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

> Martin v. Löwis wrote: >>> Hmm, perhaps when using sets as work queues? >> >> A number of comments: >> >> - it's somewhat confusing to use a set as a *queue*, given >>   that it won't provide FIFO semantics. >> - there are more appropriate and direct container structures >>   available, including

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Martin v. Löwis wrote: >> Hmm, perhaps when using sets as work queues? > > A number of comments: > > - it's somewhat confusing to use a set as a *queue*, given > that it won't provide FIFO semantics. > - there are more appropriate and direct contai

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Mon, 26 Oct 2009 05:43:52 am Martin v. Löwis wrote: > What Alexander wants is that the set type also becomes useful for > storing canonical representations. I find that inappropriate, since > dicts already provide the one obvious way to do it. Only because dicts came first in Python rather than

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

> Hmm, perhaps when using sets as work queues? A number of comments: - it's somewhat confusing to use a set as a *queue*, given that it won't provide FIFO semantics. - there are more appropriate and direct container structures available, including a dedicated Queue module (even though this

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

> If you actually care about that aspect of the semantics for a particular > application, it would be far clearer to use a full dict() and live with > the additional memory usage. While I can see how saving that > pointer-per-entry might make sense in some circumstances, for most I > would see it a

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

> Given Alexander's premise, I agree with your response. But his premise > is wrong. Python's current builtin set class maps abstract equivalence > classes to representative members. And this *is* useful. Mapping > arbitrary members of such classes to representative members, sometimes > called 'can

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

* James Y. Knight: > On Oct 25, 2009, at 2:50 AM, Terry Reedy wrote: > >> Alex Martelli wrote: >>> Next(s) would seem good... >> >> That does not work. It has to be next(iter(s)), and that has been >> tried and eliminated because it is significantly slower. > > But who cares about the speed of get

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Oct 25, 2009, at 2:50 AM, Terry Reedy wrote: Alex Martelli wrote: Next(s) would seem good... That does not work. It has to be next(iter(s)), and that has been tried and eliminated because it is significantly slower. But who cares about the speed of getting an arbitrary element from a

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Alex Martelli wrote: Next(s) would seem good... That does not work. It has to be next(iter(s)), and that has been tried and eliminated because it is significantly slower. Interesting. It depends a bit on the speed of tuple unpacking, but presumably that is quite fast. On my system it is pre

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Terry Reedy wrote: >> In this model, a >> get() method, or even a __getitem__ with s[k] is k, is only natural. > > No, if key and value were the same thing, the get method would be > nonesense, as Guido said. But since they are not, since the implict key > is an abstract class, retrieving the rep

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Next(s) would seem good... Alex Sent from my iPhone On Oct 24, 2009, at 6:47 PM, John Arbash Meinel > wrote: Adam Olsen wrote: On Fri, Oct 23, 2009 at 11:04, Vitor Bosshard wrote: I see this as being useful for frozensets as well, where you can't get an arbitrary element easily due to

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Adam Olsen wrote: > On Fri, Oct 23, 2009 at 11:04, Vitor Bosshard wrote: >> I see this as being useful for frozensets as well, where you can't get >> an arbitrary element easily due to the obvious lack of .pop(). I ran >> into this recently, when I had a frozenset that I knew had 1 element >> (it

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Fri, Oct 23, 2009 at 11:04, Vitor Bosshard wrote: > I see this as being useful for frozensets as well, where you can't get > an arbitrary element easily due to the obvious lack of .pop(). I ran > into this recently, when I had a frozenset that I knew had 1 element > (it was the difference betwe

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Alexander Belopolsky wrote: On Fri, Oct 23, 2009 at 6:46 PM, Steven D'Aprano wrote: On Sat, 24 Oct 2009 06:04:12 am Terry Reedy wrote: .. fwiw, I think the use case for this is sufficiently rare that it does not need a separate method just for this purpose. And yet it keeps coming up, again

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Guido van Rossum wrote: On Sat, Oct 24, 2009 at 10:34 AM, Alexander Belopolsky wrote: To me, however, a set seems to be a container that is a specialization of a dict with values and keys being the same. That's absurd; the mapping provides nothing useful. Given Alexander's premise, I agree

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Steven D'Aprano wrote: On Sat, 24 Oct 2009 08:12:26 pm Martin (gzlist) wrote: There's a different proposed meaning for `set.get` that's been discussed on python-dev before: For that case, I think the OP was mistaken mistak

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, Oct 24, 2009 at 10:34 AM, Alexander Belopolsky wrote: > To me, however, a set seems to be a container that is a specialization > of a dict with values and keys being the same. That's absurd; the mapping provides nothing useful. -- --Guido van Rossum PS. My elbow needs a couple more wee

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Fri, Oct 23, 2009 at 6:46 PM, Steven D'Aprano wrote: > On Sat, 24 Oct 2009 06:04:12 am Terry Reedy wrote: .. >> fwiw, I think the use case for this is sufficiently rare that it does >> not need a separate method just for this purpose. > > > And yet it keeps coming up, again and again... obvious

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Hi, someone on this list mentioned that much of the s.get() time is spend on the name lookup for get(). That is indeed the case: === from timeit import * stats = ["for i in xrange(1000): iter(s).next() ", "for i in xrange(1000): \n\tfor x in s: \n\t\tbreak",

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, 24 Oct 2009 08:12:26 pm Martin (gzlist) wrote: > On 24/10/2009, Nick Coghlan wrote: > > Ben Finney wrote: > >> Which then raises the question “what part of the set does it > >> get?”, which the function signature does nothing to answer. I'm > >> proposing that a no-parameters ‘set.get’ is

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Hi, I agree. But, here are the pros/cons collected from the recent list repsonses: Pro: - more readable - newbies will encounter one of the fastest solution (.get()) before trying slower "first solutions" like (iter(set).next()) Cons: - no name consensus. get() getany() arbitrary() ? - BDFL

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On 24/10/2009, Nick Coghlan wrote: > Ben Finney wrote: >> Which then raises the question “what part of the set does it get?”, >> which the function signature does nothing to answer. I'm proposing that >> a no-parameters ‘set.get’ is needlessly confusing to think about. > > The fact that set.get()

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Ben Finney wrote: > Which then raises the question “what part of the set does it get?”, > which the function signature does nothing to answer. I'm proposing that > a no-parameters ‘set.get’ is needlessly confusing to think about. The fact that set.get() is just set.pop() without removing the resul

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Ben Finney writes: > Steven D'Aprano writes: > > "get" is such a generic term that I don't believe that is a problem. > > The problem above is made less problematic by the fact that the function > signature (e.g. 'foo_dict.get(key)') clarifies the answer to the > question "get what?". Wher

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Steven D'Aprano writes: > On Sat, 24 Oct 2009 02:02:48 pm Ben Finney wrote: > > I would expect a parameter-less ‘set.get’ to get the set. Not > > terribly useful, but the name and function signature doesn't suggest > > anything else. > > You already have the set. Why would you need a method that

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, 24 Oct 2009 02:02:48 pm Ben Finney wrote: > Steven D'Aprano writes: > > On Sat, 24 Oct 2009 10:26:27 am Ben Finney wrote: > > > Steven D'Aprano writes: > > > > The lack of get() in sets and frozensets is sounding more and > > > > more to me like the victory of purity over practicality. >

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Steven D'Aprano writes: > On Sat, 24 Oct 2009 10:26:27 am Ben Finney wrote: > > Steven D'Aprano writes: > > > The lack of get() in sets and frozensets is sounding more and more > > > to me like the victory of purity over practicality. > > > > What would be the input to ‘set.get’? > > It wouldn't

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, 24 Oct 2009 10:26:27 am Ben Finney wrote: > Steven D'Aprano writes: > > On Sat, 24 Oct 2009 06:04:12 am Terry Reedy wrote: > > > fwiw, I think the use case for this is sufficiently rare that it > > > does not need a separate method just for this purpose. > > > > And yet it keeps coming up,

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Steven D'Aprano writes: > On Sat, 24 Oct 2009 06:04:12 am Terry Reedy wrote: > > fwiw, I think the use case for this is sufficiently rare that it > > does not need a separate method just for this purpose. > > And yet it keeps coming up, again and again... obviously people using > sets in code thi

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, 24 Oct 2009 07:53:24 am Willi Richert wrote: > Hi, > > surprised about the performance of for/break provided by Vitor, I did > some more testing. It revealed that indeed we can forget the get() > (which was implemented as a stripped down pop()): I don't understand that conclusion. Accord

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, 24 Oct 2009 06:04:12 am Terry Reedy wrote: > John Arbash Meinel wrote: > > So 'for x in s: break' is about 2x faster than next(iter(s)) and 3x > > faster than (iter(s).next()). > > I was pretty surprised that it was 30% faster than "for x in s: > > pass". I assume it has something to do wit

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Sat, 24 Oct 2009 07:11:57 am John Arbash Meinel wrote: > The point of my test was that it was a set with a *single* item, and > 'break' was 30% faster than 'pass'. Which was surprising. Not really. See below. > Certainly > the difference is huge if there are 10k items in the set. Earlier

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Hi, surprised about the performance of for/break provided by Vitor, I did some more testing. It revealed that indeed we can forget the get() (which was implemented as a stripped down pop()): from timeit import * stats = ["for i in xrange(1000): iter(s).next() ", "for i in xrange(100

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Terry Reedy wrote: > John Arbash Meinel wrote: >> So 'for x in s: break' is about 2x faster than next(iter(s)) and 3x >> faster than (iter(s).next()). >> I was pretty surprised that it was 30% faster than "for x in s: pass". I >> assume it has something to do with a potential "else:" statement? >

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

John Arbash Meinel wrote: So 'for x in s: break' is about 2x faster than next(iter(s)) and 3x faster than (iter(s).next()). I was pretty surprised that it was 30% faster than "for x in s: pass". I assume it has something to do with a potential "else:" statement? for x in s: pass iterates throu

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Willi Richert wrote: > recently I wrote an algorithm, in which very often I had to get an arbitrary > element from a set without removing it. See this discussion: http://comments.gmane.org/gmane.comp.python.ideas/5606 Stefan ___ Python-Dev mailing li

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

2009/10/23 John Arbash Meinel : > I was pretty surprised that it was 30% faster than "for x in s: pass". I > assume it has something to do with a potential "else:" statement? I'd imagine it's actually because it has to call next() a second time and deal with the StopIteration exception - the loop

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

Vitor Bosshard wrote: > 2009/10/23 Willi Richert : >> Hi, >> >> recently I wrote an algorithm, in which very often I had to get an arbitrary >> element from a set without removing it. >> >> Three possibilities came to mind: >> >> 1. >> x = some_set.pop() >> some_set.add(x) >> >> 2. >> for x in some

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

2009/10/23 Willi Richert : > Hi, > > recently I wrote an algorithm, in which very often I had to get an arbitrary > element from a set without removing it. > > Three possibilities came to mind: > > 1. > x = some_set.pop() > some_set.add(x) > > 2. > for x in some_set: >        break > > 3. > x = ite

Re: [Python-Dev] Retrieve an arbitrary element from a set without removing it

On Fri, 23 Oct 2009 08:32:45 pm Willi Richert wrote: > Hi, > > recently I wrote an algorithm, in which very often I had to get an > arbitrary element from a set without removing it. > > Three possibilities came to mind: ... > Of course, the third should be the fastest. If you need one or more item

[Python-Dev] Retrieve an arbitrary element from a set without removing it

Hi, recently I wrote an algorithm, in which very often I had to get an arbitrary element from a set without removing it. Three possibilities came to mind: 1. x = some_set.pop() some_set.add(x) 2. for x in some_set: break 3. x = iter(some_set).next() Of course, the third should be