Issue1212091
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Created on 2005-05-31 15:54 by mkc, last changed 2022-04-11 14:56 by admin. This issue is now closed.
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msg54547 - (view) | Author: Mike Coleman (mkc) | Date: 2005-05-31 15:54 | |
It would be nice to have an "arbitrary element" method that would return some arbitrary element of a non-empty set (throwing an exception if the set is empty). Something like this >>> s = sets.Set([1,2,3]) >>> print s.element() 2 AFAIK, the current alternative would be to do something like this >>> print list(s)[0] which is somewhat expensive and unreadable. It'd be fine if the operator returned the same or a different element each time. Perhaps it'd be useful if it returned the same element each time for frozen sets. |
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msg54548 - (view) | Author: Georg Brandl (georg.brandl) * | Date: 2005-05-31 15:59 | |
Logged In: YES user_id=1188172 For mutable sets, I think that pop() is better suited. Do you have a use case of your proposal with frozensets? |
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msg54549 - (view) | Author: Raymond Hettinger (rhettinger) * | Date: 2005-05-31 16:54 | |
Logged In: YES user_id=80475 I had looked at putting in a choose() method but there were a shortage of use cases that were not better served by pop() or by iteration. Will leave this open as the request may yet prove its worth. To do so, I would need to see examples of practical code where choose() is a better choice than existing alternatives. For the record, here were some of the lines of thinking about choose(). * Given that any element could be returned, it was logical to return the first encountered. That led to the odd situation where the method would return the same value on every call (unless followed by a set mutation) making the method useless in a loop. * If needed, an efficient alternative is available: iter(s).next(). And here is a more readable, encapsulated version that a programmer could dash off in seconds: def choose(s, default=None): for elem in s: return elem return default * I had looked at a few textbook algorithms that were expressed in terms of choose(). Without exception, their Python code was better expressed using pop(), In a couple of cases, the pop() needed to be followed by an add() if the item was to be left in the set. Those two cases all had other set mutations occuring on each iteration (otherwise, the pop/add pair would always get the same element). |
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msg54550 - (view) | Author: Mike Coleman (mkc) | Date: 2005-06-03 20:45 | |
Logged In: YES user_id=555 I like rhettinger's "iter(s).next()" solution quite well, except that it's kind of an obscure way to say "give me an element of this set". I'm completely okay with the idea that the choose method may or may not return the same element each time--I just want a "for example" element. To my thinking, pop() isn't really a good solution because it treats sets and frozensets differently. Plus a pop()/add() cycle seems wasteful and confusing. One could argue that in the other place where pop() is available (lists), there is also a simple way to "peek" at what would be popped, which is kind of what we're talking about here. As for a use case, I'm not sure I have a great one. This came up when I was writing some union/find code. I was using sets to represent equivalence classes and I wanted to be able to grab a representative of a set (which would be any element of the set), given the set. I was surprised to find that there didn't seem to be a straightforward way to do this. I agree that I could just copy your 'choose' function into my code easily. There are several such functions that I already copy into pretty much every script I write these days (warn, error, and groupby, which probably do just what you think they do). If it's something that would be broadly useful, though, it seems like it'd be a lot more efficient for everyone to know the function by the same name (i.e., by having it be in the standard library). |
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msg54551 - (view) | Author: Raymond Hettinger (rhettinger) * | Date: 2005-06-14 08:41 | |
Logged In: YES user_id=80475 Closed due to lack of use cases and my deep suspicion that it is the wrong thing to do. |
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msg54552 - (view) | Author: Michael Hudson (mwh) | Date: 2005-06-14 10:14 | |
Logged In: YES user_id=6656 Just a comment that I've actually wanted this function recently; in the case where the set has a single element, it can be interesting to know what that element is. It's not a big thing, iter(s).next() works, but it's obscure. |
History | |||
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Date | User | Action | Args |
2022-04-11 14:56:11 | admin | set | github: 42033 |
2005-05-31 15:54:41 | mkc | create |